Joint chroma residual video coding

ABSTRACT

A video decoder can be configured to receive, for a first chroma component of a block of the video data, information for first residual samples that correspond to a difference between a first chroma block of the first chroma component and a first prediction block of the first chroma component; determine intermediate reconstructed samples based on the first residual samples; receive, for a second chroma component of the block of video data, information for second residual samples that correspond to a difference between a second chroma block of the second chroma component and the intermediate reconstructed samples; reconstruct the first chroma block based on the first residual samples and the first prediction block; reconstruct the second chroma block based on the second residual samples and the intermediate reconstructed samples; and output decoded video data comprising the reconstructed first chroma block and the reconstructed second chroma block.

This application claims the benefit of U.S. Provisional PatentApplication 62/945,750, filed 9 Dec. 2019, the entire content beingincorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to video encoding and video decoding.

BACKGROUND

Digital video capabilities can be incorporated into a wide range ofdevices, including digital televisions, digital direct broadcastsystems, wireless broadcast systems, personal digital assistants (PDAs),laptop or desktop computers, tablet computers, e-book readers, digitalcameras, digital recording devices, digital media players, video gamingdevices, video game consoles, cellular or satellite radio telephones,so-called “smart phones,” video teleconferencing devices, videostreaming devices, and the like. Digital video devices implement videocoding techniques, such as those described in the standards defined byMPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced VideoCoding (AVC), ITU-T H.265/High Efficiency Video Coding (HEVC), andextensions of such standards. The video devices may transmit, receive,encode, decode, and/or store digital video information more efficientlyby implementing such video coding techniques.

Video coding techniques include spatial (intra-picture) predictionand/or temporal (inter-picture) prediction to reduce or removeredundancy inherent in video sequences. For block-based video coding, avideo slice (e.g., a video picture or a portion of a video picture) maybe partitioned into video blocks, which may also be referred to ascoding tree units (CTUs), coding units (CUs) and/or coding nodes. Videoblocks in an intra-coded (I) slice of a picture are encoded usingspatial prediction with respect to reference samples in neighboringblocks in the same picture. Video blocks in an inter-coded (P or B)slice of a picture may use spatial prediction with respect to referencesamples in neighboring blocks in the same picture or temporal predictionwith respect to reference samples in other reference pictures. Picturesmay be referred to as frames, and reference pictures may be referred toas reference frames.

SUMMARY

In general, this disclosure describes techniques for joint coding ofchroma residuals (JCCR). In JCCR, the residual chroma blocks (e.g.,difference between chroma blocks and respective chroma predictionblocks) are coded together (e.g., one residual block is coded for theresidual chroma blocks). In some examples, JCCR may result in lossycoding, and therefore JCCR may be disabled when lossless coding isenabled. However, there may be benefits to the video quality byutilizing lossless coding or at least less lossy coding. This disclosuredescribes examples of techniques for JCCR with lossless or less lossycoding, thereby allowing for the benefits of less lossy coding whileachieving at least some of the benefits (e.g., reduction in amount ofresidual information that is signaled) of JCCR.

According to one example, a method of decoding video data includesreceiving information for first residual samples for a first chromacomponent of a block of the video data, wherein the first residualsamples correspond to a difference between a first chroma block of thefirst chroma component and a first prediction block of the first chromacomponent; determining intermediate reconstructed samples based on thefirst residual samples; receiving information for second residualsamples for a second chroma component of the block of video data,wherein the second residual samples correspond to a difference between asecond chroma block of the second chroma component and the intermediatereconstructed samples; reconstructing the first chroma block based onthe first residual samples and the first prediction block;reconstructing the second chroma block based on the second residualsamples and the intermediate reconstructed samples; and outputtingdecoded video data comprising the reconstructed first chroma block andthe reconstructed second chroma block.

According to another example, a method includes determining firstresidual samples for a first chroma component of a block of the videodata, wherein the first residual samples correspond to a differencebetween a first chroma block of the first chroma component and a firstprediction block of the first chroma component; determining intermediatereconstructed samples based on the first residual samples and a secondprediction block of the second chroma component; determining secondresidual samples for the second chroma component of the block of videodata, wherein the second residual samples correspond to a differencebetween a second chroma block of the second chroma component and theintermediate reconstructed samples; and outputting, in a bitstream ofencoded video data, syntax elements representing the first residualsamples and the second residual samples.

According to another example, a device for decoding video data includesa memory configured to store the video data and one or more processorsimplemented in circuitry and configured to receive information for firstresidual samples for a first chroma component of a block of the videodata, wherein the first residual samples correspond to a differencebetween a first chroma block of the first chroma component and a firstprediction block of the first chroma component; determine intermediatereconstructed samples based on the first residual samples; receiveinformation for second residual samples for a second chroma component ofthe block of video data, wherein the second residual samples correspondto a difference between a second chroma block of the second chromacomponent and the intermediate reconstructed samples; reconstruct thefirst chroma block based on the first residual samples and the firstprediction block; reconstruct the second chroma block based on thesecond residual samples and the intermediate reconstructed samples; andoutput decoded video data comprising the reconstructed first chromablock and the reconstructed second chroma block.

According to another example, a device for encoding video data includesa memory configured to store video data and one or more processorsimplemented in circuitry and configured to determine first residualsamples for a first chroma component of a block of the video data,wherein the first residual samples correspond to a difference between afirst chroma block of the first chroma component and a first predictionblock of the first chroma component; determine intermediatereconstructed samples based on the first residual samples and a secondprediction block of the second chroma component; determine secondresidual samples for the second chroma component of the block of videodata, wherein the second residual samples correspond to a differencebetween a second chroma block of the second chroma component and theintermediate reconstructed samples; and output, in a bitstream ofencoded video data, syntax elements representing the first residualsamples and the second residual samples.

According to another example, a computer-readable storage medium storesinstructions that when executed by one or more processors cause the oneor more processors to receive information for first residual samples fora first chroma component of a block of the video data, wherein the firstresidual samples correspond to a difference between a first chroma blockof the first chroma component and a first prediction block of the firstchroma component; determine intermediate reconstructed samples based onthe first residual samples; receive information for second residualsamples for a second chroma component of the block of video data,wherein the second residual samples correspond to a difference between asecond chroma block of the second chroma component and the intermediatereconstructed samples; reconstruct the first chroma block based on thefirst residual samples and the first prediction block; reconstruct thesecond chroma block based on the second residual samples and theintermediate reconstructed samples; and output decoded video datacomprising the reconstructed first chroma block and the reconstructedsecond chroma block.

According to another example, a computer-readable storage medium storesinstructions that when executed by one or more processors cause the oneor more processors to determine first residual samples for a firstchroma component of a block of the video data, wherein the firstresidual samples correspond to a difference between a first chroma blockof the first chroma component and a first prediction block of the firstchroma component; determine intermediate reconstructed samples based onthe first residual samples and a second prediction block of the secondchroma component; determine second residual samples for the secondchroma component of the block of video data, wherein the second residualsamples correspond to a difference between a second chroma block of thesecond chroma component and the intermediate reconstructed samples; andoutput, in a bitstream of encoded video data, syntax elementsrepresenting the first residual samples and the second residual samples.

According to another example, an apparatus for decoding video dataincludes means for receiving information for first residual samples fora first chroma component of a block of the video data, wherein the firstresidual samples correspond to a difference between a first chroma blockof the first chroma component and a first prediction block of the firstchroma component; means for determining intermediate reconstructedsamples based on the first residual samples; means for receivinginformation for second residual samples for a second chroma component ofthe block of video data, wherein the second residual samples correspondto a difference between a second chroma block of the second chromacomponent and the intermediate reconstructed samples; means forreconstructing the first chroma block based on the first residualsamples and the first prediction block; means for reconstructing thesecond chroma block based on the second residual samples and theintermediate reconstructed samples; and means for outputting decodedvideo data comprising the reconstructed first chroma block and thereconstructed second chroma block.

According to another example, an apparatus for encoding video dataincludes means for determining first residual samples for a first chromacomponent of a block of the video data, wherein the first residualsamples correspond to a difference between a first chroma block of thefirst chroma component and a first prediction block of the first chromacomponent; means for determining intermediate reconstructed samplesbased on the first residual samples and a second prediction block of thesecond chroma component; means for determining second residual samplesfor the second chroma component of the block of video data, wherein thesecond residual samples correspond to a difference between a secondchroma block of the second chroma component and the intermediatereconstructed samples; and means for outputting, in a bitstream ofencoded video data, syntax elements representing the first residualsamples and the second residual samples.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system that may perform the techniques of this disclosure.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure, and a corresponding coding tree unit(CTU).

FIG. 3 is a block diagram illustrating an example video encoder that mayperform the techniques of this disclosure.

FIG. 4 is a block diagram illustrating an example video decoder that mayperform the techniques of this disclosure.

FIG. 5 is a flowchart illustrating an example of encoding video data.

FIG. 6 is a flowchart illustrating an example of decoding video data.

FIG. 7 is a flowchart illustrating an example of encoding video data.

FIG. 8 is a flowchart illustrating an example of decoding video data.

DETAILED DESCRIPTION

Video coding (e.g., video encoding and/or video decoding) typicallyinvolves predicting a block of video data from either an already codedblock of video data in the same picture (e.g., intra prediction) or analready coded block of video data in a different picture (e.g., interprediction). In some instances, the video encoder also calculatesresidual data by comparing the prediction block to the original block.Thus, the residual data represents a difference between the predictionblock and the original block. To reduce the number of bits needed tosignal the residual data, the video encoder transforms and quantizes theresidual data and signals the transformed and quantized residual data inthe encoded bitstream. The compression achieved by the transform andquantization processes may be lossy, meaning that transform andquantization processes may introduce distortion into the decoded videodata.

A video decoder decodes and adds the residual data to the predictionblock to produce a reconstructed video block that matches the originalvideo block more closely than the prediction block alone. Due to theloss introduced by the transforming and quantizing of the residual data,the first reconstructed block may have distortion or artifacts. Onecommon type of artifact or distortion is referred to as blockiness,where the boundaries of the blocks used to code the video data arevisible.

To further improve the quality of decoded video, a video decoder canperform one or more filtering operations on the reconstructed videoblocks. Examples of these filtering operations include deblockingfiltering, sample adaptive offset (SAO) filtering, and adaptive loopfiltering (ALF). Parameters for these filtering operations may either bedetermined by a video encoder and explicitly signaled in the encodedvideo bitstream or may be implicitly determined by a video decoderwithout needing the parameters to be explicitly signaled in the encodedvideo bitstream.

As will be explained in more detail below, some video coding standardssupport lossless coding modes. In a lossless coding mode, the decodedvideo data matches the encoded video data without error or distortion.One technique for achieving lossless video coding is to skip thequantization and inverse quantization processes described above. As willbe explained in more detail below, video data is frequently coded inblocks of luma samples with two corresponding blocks of chroma samples.The video data may be coded in a joint chroma mode, also referred to asjoint coding of chroma residuals (JCCR) mode, where a single chromaresidual block is encoded for the two corresponding blocks of chromasamples. Existing video standards do not currently implement losslesscoding modes that can also be used in conjunction with a joint chromamode.

This disclosure describes techniques for implementing JCCR modes thatalso support lossless coding. Implementing lossless coding with JCCRmodes, as described in this disclosure, may for some coding scenariosadvantageously result in improved compression compared to otheravailable lossless coding techniques.

In video coding, for YUV (e.g., Y, Cb, Cr) format, there may be twochroma components (e.g., Cb and Cr). For ease, examples are describedherein with respect to Cb and Cr blocks, but the techniques areextendable to other types of chroma blocks and other video formats(e.g., RGB). For video encoding, a video encoder determines a residualblock for the Cb block, referred to as resCb, and the Cr block, referredto as resCr. ResCb and resCr may represent a difference betweenrespective chroma prediction blocks and original chroma blocks. That is,resCb represents a difference between the original Cb block and the Cbprediction block, and resCr represents a difference between the originalCr block and the Cr prediction block. In some techniques, the videoencoder signals information for resCb and separately signals informationfor resCr, for example, after one or more of transformation to transformor frequency domain, quantization, and context-based or bypass-coding. Avideo decoder receives information for resCb′ and resCr′, separately,and adds resCb and resCr to respective ones of the Cb prediction blockand Cr prediction block to reconstruct the Cb block and the Cr block. Inlossy coding, for resCb′ and resCr′ may not exactly equal for resCb andresCr due to loss caused by the quantization and dequantizationprocesses. In some implementations of lossless coding, resCb′ and resCr′may equal resCb and resCr.

In a JCCR mode, rather than coding resCb and resCr separately, the videoencoder constructs a joint residual block, referred to as resJointCbased on resCb and resCr. The video encoder signals information forresJointC, which may reduce the amount of information that needs to besignaled as compared to separately signaling information for resCb andresCr. The video decoder receives information for resJointC, generatesresJointC from the received information (referred to as resJointC′ toindicate that resJointC′ is at the video decoder side), and thengenerates resCb′ and resCr′.

In some cases, the use of JCCR may result in lossy coding. For instance,resCb′ (e.g., residual Cb block at video decoder side) may not beidentical to resCb (e.g., residual Cb block at video encoder side), andresCr′ (e.g., residual Cr block at video decoder side) may not beidentical to resCr (e.g., residual Cr block at video decoder side). Useof JCCR may result in lossy coding due to the transformation andquantization operations performed by the video encoder. However, in somecases, the operations used to generate the joint residual chroma block(e.g., resJointC) may itself by lossy. Techniques in which JCCR resultsin lossy coding may be referred to as a first type of JCCR.

For higher video quality, there may be instances where lossless videocoding is preferred. Because in some examples JCCR is lossy, losslessvideo coding may not be available. Also, where lossless video coding maybe utilized, the benefits from JCCR (e.g., reduction of amount ofresidual information that is signaled) may not be available.

This disclosure describes examples of techniques of a second type ofJCCR. The second type of JCCR may be lossless or may be less lossy thanthe first type of JCCR. In some examples, the second type of JCCR may beutilized where transformation and/or quantization is bypassed or wheretransformation and/or quantization process is lossless. However, theexamples of the second type of JCCR should not be construed to belimited to examples where there is no loss or limited to examples wheretransformation and/or quantization is bypassed or where transformationand/or quantization process is lossless.

According to the techniques of this disclosure, a video decoder may beconfigured to receive information for first residual samples for a firstchroma component of a block of the video data and reconstruct the firstchroma block based on the first residual samples and a first predictionblock. The first residual samples may be determined in a manner suchthat adding the first residual samples to samples of the firstprediction block results in the original first chroma block, and henceresults in lossless coding of the first chroma block.

The video decoder may also determine intermediate reconstructed samplesfor a second chroma component of the block of video data based on thefirst residual samples and a second prediction block. The video decodermay receive information for second residual samples and reconstruct thesecond chroma block by adding the second residual samples to theintermediate reconstructed samples. The second residual samples may bedetermined in a manner such that adding the second residual samples tothe intermediate reconstructed samples results in the original secondchroma block, and hence results in lossless coding of the second chromablock. Moreover, as the intermediate reconstructed samples are typicallyvery close to the samples of the original second chroma block, thesecond residual samples may typically be relatively small values, withlots of 0's and 1's for example, and thus can be efficiently coded withrelatively few bits. Thus, by reconstructing the second chroma blockbased on the second residual samples and the intermediate reconstructedsamples, a video encoding and decoding system may advantageouslyimplement lossless coding while still benefiting from the bit savingsthat JCCR produces for some coding scenarios.

FIG. 1 is a block diagram illustrating an example video encoding anddecoding system 100 that may perform the techniques of this disclosure.The techniques of this disclosure are generally directed to coding(encoding and/or decoding) video data. In general, video data includesany data for processing a video. Thus, video data may include raw,unencoded video, encoded video, decoded (e.g., reconstructed) video, andvideo metadata, such as signaling data.

As shown in FIG. 1, system 100 includes a source device 102 thatprovides encoded video data to be decoded and displayed by a destinationdevice 116, in this example. In particular, source device 102 providesthe video data to destination device 116 via a computer-readable medium110. Source device 102 and destination device 116 may comprise any of awide range of devices, including desktop computers, notebook (i.e.,laptop) computers, tablet computers, set-top boxes, telephone handsetssuch as smartphones, televisions, cameras, display devices, digitalmedia players, video gaming consoles, video streaming device, or thelike. In some cases, source device 102 and destination device 116 may beequipped for wireless communication, and thus may be referred to aswireless communication devices.

In the example of FIG. 1, source device 102 includes video source 104,memory 106, video encoder 200, and output interface 108. Destinationdevice 116 includes input interface 122, video decoder 300, memory 120,and display device 118. In accordance with this disclosure, videoencoder 200 of source device 102 and video decoder 300 of destinationdevice 116 may be configured to apply the techniques for joint coding ofchroma residuals (JCCR). Thus, source device 102 represents an exampleof a video encoding device, while destination device 116 represents anexample of a video decoding device. In other examples, a source deviceand a destination device may include other components or arrangements.For example, source device 102 may receive video data from an externalvideo source, such as an external camera. Likewise, destination device116 may interface with an external display device, rather than includean integrated display device. In this disclosure, operations describedas being performed at the video encoder side may, for example, beperformed by video encoder 200 of system 100, while operations describedas being performed at the video decoder side may, for example, beperformed by video decoder 300 of system 100.

System 100 as shown in FIG. 1 is merely one example. In general, anydigital video encoding and/or decoding device may perform techniques forJCCR. Source device 102 and destination device 116 are merely examplesof such coding devices in which source device 102 generates coded videodata for transmission to destination device 116. This disclosure refersto a “coding” device as a device that performs coding (encoding and/ordecoding) of data. Thus, video encoder 200 and video decoder 300represent examples of coding devices, in particular, a video encoder anda video decoder, respectively. In some examples, source device 102 anddestination device 116 may operate in a substantially symmetrical mannersuch that each of source device 102 and destination device 116 includesvideo encoding and decoding components. Hence, system 100 may supportone-way or two-way video transmission between source device 102 anddestination device 116, e.g., for video streaming, video playback, videobroadcasting, or video telephony.

In general, video source 104 represents a source of video data (i.e.,raw, unencoded video data) and provides a sequential series of pictures(also referred to as “frames”) of the video data to video encoder 200,which encodes data for the pictures. Video source 104 of source device102 may include a video capture device, such as a video camera, a videoarchive containing previously captured raw video, and/or a video feedinterface to receive video from a video content provider. As a furtheralternative, video source 104 may generate computer graphics-based dataas the source video, or a combination of live video, archived video, andcomputer-generated video. In each case, video encoder 200 encodes thecaptured, pre-captured, or computer-generated video data. Video encoder200 may rearrange the pictures from the received order (sometimesreferred to as “display order”) into a coding order for coding. Videoencoder 200 may generate a bitstream including encoded video data.Source device 102 may then output the encoded video data via outputinterface 108 onto computer-readable medium 110 for reception and/orretrieval by, e.g., input interface 122 of destination device 116.

Memory 106 of source device 102 and memory 120 of destination device 116represent general purpose memories. In some examples, memories 106, 120may store raw video data, e.g., raw video from video source 104 and raw,decoded video data from video decoder 300. Additionally oralternatively, memories 106, 120 may store software instructionsexecutable by, e.g., video encoder 200 and video decoder 300,respectively. Although memory 106 and memory 120 are shown separatelyfrom video encoder 200 and video decoder 300 in this example, it shouldbe understood that video encoder 200 and video decoder 300 may alsoinclude internal memories for functionally similar or equivalentpurposes. Furthermore, memories 106, 120 may store encoded video data,e.g., output from video encoder 200 and input to video decoder 300. Insome examples, portions of memories 106, 120 may be allocated as one ormore video buffers, e.g., to store raw, decoded, and/or encoded videodata.

Computer-readable medium 110 may represent any type of medium or devicecapable of transporting the encoded video data from source device 102 todestination device 116. In one example, computer-readable medium 110represents a communication medium to enable source device 102 totransmit encoded video data directly to destination device 116 inreal-time, e.g., via a radio frequency network or computer-basednetwork. Output interface 108 may modulate a transmission signalincluding the encoded video data, and input interface 122 may demodulatethe received transmission signal, according to a communication standard,such as a wireless communication protocol. The communication medium maycomprise any wireless or wired communication medium, such as a radiofrequency (RF) spectrum or one or more physical transmission lines. Thecommunication medium may form part of a packet-based network, such as alocal area network, a wide-area network, or a global network such as theInternet. The communication medium may include routers, switches, basestations, or any other equipment that may be useful to facilitatecommunication from source device 102 to destination device 116.

In some examples, source device 102 may output encoded data from outputinterface 108 to storage device 112. Similarly, destination device 116may access encoded data from storage device 112 via input interface 122.Storage device 112 may include any of a variety of distributed orlocally accessed data storage media such as a hard drive, Blu-ray discs,DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or anyother suitable digital storage media for storing encoded video data.

In some examples, source device 102 may output encoded video data tofile server 114 or another intermediate storage device that may storethe encoded video generated by source device 102. Destination device 116may access stored video data from file server 114 via streaming ordownload. File server 114 may be any type of server device capable ofstoring encoded video data and transmitting that encoded video data tothe destination device 116. File server 114 may represent a web server(e.g., for a website), a File Transfer Protocol (FTP) server, a contentdelivery network device, or a network attached storage (NAS) device.Destination device 116 may access encoded video data from file server114 through any standard data connection, including an Internetconnection. This may include a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., digital subscriber line (DSL),cable modem, etc.), or a combination of both that is suitable foraccessing encoded video data stored on file server 114. File server 114and input interface 122 may be configured to operate according to astreaming transmission protocol, a download transmission protocol, or acombination thereof.

Output interface 108 and input interface 122 may represent wirelesstransmitters/receivers, modems, wired networking components (e.g.,Ethernet cards), wireless communication components that operateaccording to any of a variety of IEEE 802.11 standards, or otherphysical components. In examples where output interface 108 and inputinterface 122 comprise wireless components, output interface 108 andinput interface 122 may be configured to transfer data, such as encodedvideo data, according to a cellular communication standard, such as 4G,4G-LTE (Long-Term Evolution), LTE Advanced, 5G, or the like. In someexamples where output interface 108 comprises a wireless transmitter,output interface 108 and input interface 122 may be configured totransfer data, such as encoded video data, according to other wirelessstandards, such as an IEEE 802.11 specification, an IEEE 802.15specification (e.g., ZigBee™), a Bluetooth™ standard, or the like. Insome examples, source device 102 and/or destination device 116 mayinclude respective system-on-a-chip (SoC) devices. For example, sourcedevice 102 may include an SoC device to perform the functionalityattributed to video encoder 200 and/or output interface 108, anddestination device 116 may include an SoC device to perform thefunctionality attributed to video decoder 300 and/or input interface122.

The techniques of this disclosure may be applied to video coding insupport of any of a variety of multimedia applications, such asover-the-air television broadcasts, cable television transmissions,satellite television transmissions, Internet streaming videotransmissions, such as dynamic adaptive streaming over HTTP (DASH),digital video that is encoded onto a data storage medium, decoding ofdigital video stored on a data storage medium, or other applications.

Input interface 122 of destination device 116 receives an encoded videobitstream from computer-readable medium 110 (e.g., a communicationmedium, storage device 112, file server 114, or the like). The encodedvideo bitstream may include signaling information defined by videoencoder 200, which is also used by video decoder 300, such as syntaxelements having values that describe characteristics and/or processingof video blocks or other coded units (e.g., slices, pictures, groups ofpictures, sequences, or the like). Display device 118 displays decodedpictures of the decoded video data to a user. Display device 118 mayrepresent any of a variety of display devices such as a cathode ray tube(CRT), a liquid crystal display (LCD), a plasma display, an organiclight emitting diode (OLED) display, or another type of display device.

Although not shown in FIG. 1, in some examples, video encoder 200 andvideo decoder 300 may each be integrated with an audio encoder and/oraudio decoder, and may include appropriate MUX-DEMUX units, or otherhardware and/or software, to handle multiplexed streams including bothaudio and video in a common data stream. If applicable, MUX-DEMUX unitsmay conform to the ITU H.223 multiplexer protocol, or other protocolssuch as the user datagram protocol (UDP).

Video encoder 200 and video decoder 300 each may be implemented as anyof a variety of suitable encoder and/or decoder circuitry, such as oneor more microprocessors, digital signal processors (DSPs), applicationspecific integrated circuits (ASICs), field programmable gate arrays(FPGAs), discrete logic, software, hardware, firmware or anycombinations thereof. When the techniques are implemented partially insoftware, a device may store instructions for the software in asuitable, non-transitory computer-readable medium and execute theinstructions in hardware using one or more processors to perform thetechniques of this disclosure. Each of video encoder 200 and videodecoder 300 may be included in one or more encoders or decoders, eitherof which may be integrated as part of a combined encoder/decoder (CODEC)in a respective device. A device including video encoder 200 and/orvideo decoder 300 may comprise an integrated circuit, a microprocessor,and/or a wireless communication device, such as a cellular telephone.

Examples of video coding standards include ITU-T H.261, ISO/IEC MPEG-1Visual, ITU-T H.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IECMPEG-4 Visual (MPEG-4 Part 2), ITU-T H.264 (also known as ISO/IEC MPEG-4AVC), including its Scalable Video Coding (SVC) and Multiview VideoCoding (MVC) extensions and ITU-T H.265 (also known as ISO/IEC MPEG-4HEVC) with its extensions.

Video encoder 200 and video decoder 300 may operate according to a videocoding standard, such as ITU-T H.265, also referred to as HighEfficiency Video Coding (HEVC) or extensions thereto, such as themulti-view and/or scalable video coding extensions. Alternatively, videoencoder 200 and video decoder 300 may operate according to otherproprietary or industry standards, such as the Joint Exploration TestModel (JEM) or ITU-T H.266, also referred to as Versatile Video Coding(VVC). A recent draft of the VVC standard is described in Bross, et al.“Versatile Video Coding (Draft 7),” Joint Video Experts Team (WET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 16^(th) Meeting: Geneva,CH, 1-11 Oct. 2019, JVET-P2001-v14 (hereinafter “VVC Draft 7”). Thetechniques of this disclosure, however, are not limited to anyparticular coding standard.

In general, video encoder 200 and video decoder 300 may performblock-based coding of pictures. The term “block” generally refers to astructure including data to be processed (e.g., encoded, decoded, orotherwise used in the encoding and/or decoding process). For example, ablock may include a two-dimensional matrix of samples of luminanceand/or chrominance data. In general, video encoder 200 and video decoder300 may code video data represented in a YUV (e.g., Y, Cb, Cr) format.That is, rather than coding red, green, and blue (RGB) data for samplesof a picture, video encoder 200 and video decoder 300 may code luminanceand chrominance components, where the chrominance components may includeboth red hue and blue hue chrominance components. In some examples,video encoder 200 converts received RGB formatted data to a YUVrepresentation prior to encoding, and video decoder 300 converts the YUVrepresentation to the RGB format. Alternatively, pre- andpost-processing units (not shown) may perform these conversions.

This disclosure may generally refer to coding (e.g., encoding anddecoding) of pictures to include the process of encoding or decodingdata of the picture. Similarly, this disclosure may refer to coding ofblocks of a picture to include the process of encoding or decoding datafor the blocks, e.g., prediction and/or residual coding. An encodedvideo bitstream generally includes a series of values for syntaxelements representative of coding decisions (e.g., coding modes) andpartitioning of pictures into blocks. Thus, references to coding apicture or a block should generally be understood as coding values forsyntax elements forming the picture or block.

HEVC defines various blocks, including coding units (CUs), predictionunits (PUs), and transform units (TUs). According to HEVC, a video coder(such as video encoder 200) partitions a coding tree unit (CTU) into CUsaccording to a quadtree structure. That is, the video coder partitionsCTUs and CUs into four equal, non-overlapping squares, and each node ofthe quadtree has either zero or four child nodes. Nodes without childnodes may be referred to as “leaf nodes,” and CUs of such leaf nodes mayinclude one or more PUs and/or one or more TUs. The video coder mayfurther partition PUs and TUs. For example, in HEVC, a residual quadtree(RQT) represents partitioning of TUs. In HEVC, PUs representinter-prediction data, while TUs represent residual data. CUs that areintra-predicted include intra-prediction information, such as anintra-mode indication.

As another example, video encoder 200 and video decoder 300 may beconfigured to operate according to JEM or VVC. According to JEM or VVC,a video coder (such as video encoder 200) partitions a picture into aplurality of coding tree units (CTUs). Video encoder 200 may partition aCTU according to a tree structure, such as a quadtree-binary tree (QTBT)structure or Multi-Type Tree (MTT) structure. The QTBT structure removesthe concepts of multiple partition types, such as the separation betweenCUs, PUs, and TUs of HEVC. A QTBT structure includes two levels: a firstlevel partitioned according to quadtree partitioning, and a second levelpartitioned according to binary tree partitioning. A root node of theQTBT structure corresponds to a CTU. Leaf nodes of the binary treescorrespond to coding units (CUs).

In an MTT partitioning structure, blocks may be partitioned using aquadtree (QT) partition, a binary tree (BT) partition, and one or moretypes of triple tree (TT) (also called ternary tree (TT)) partitions. Atriple or ternary tree partition is a partition where a block is splitinto three sub-blocks. In some examples, a triple or ternary treepartition divides a block into three sub-blocks without dividing theoriginal block through the center. The partitioning types in MTT (e.g.,QT, BT, and TT), may be symmetrical or asymmetrical.

In some examples, video encoder 200 and video decoder 300 may use asingle QTBT or MTT structure to represent each of the luminance andchrominance components, while in other examples, video encoder 200 andvideo decoder 300 may use two or more QTBT or MTT structures, such asone QTBT/MTT structure for the luminance component and another QTBT/MTTstructure for both chrominance components (or two QTBT/MTT structuresfor respective chrominance components).

Video encoder 200 and video decoder 300 may be configured to usequadtree partitioning per HEVC, QTBT partitioning, MTT partitioning, orother partitioning structures. For purposes of explanation, thedescription of the techniques of this disclosure is presented withrespect to QTBT partitioning. However, it should be understood that thetechniques of this disclosure may also be applied to video codersconfigured to use quadtree partitioning, or other types of partitioningas well.

The blocks (e.g., CTUs or CUs) may be grouped in various ways in apicture. As one example, a brick may refer to a rectangular region ofCTU rows within a particular tile in a picture. A tile may be arectangular region of CTUs within a particular tile column and aparticular tile row in a picture. A tile column refers to a rectangularregion of CTUs having a height equal to the height of the picture and awidth specified by syntax elements (e.g., such as in a picture parameterset). A tile row refers to a rectangular region of CTUs having a heightspecified by syntax elements (e.g., such as in a picture parameter set)and a width equal to the width of the picture.

In some examples, a tile may be partitioned into multiple bricks, eachof which may include one or more CTU rows within the tile. A tile thatis not partitioned into multiple bricks may also be referred to as abrick. However, a brick that is a true subset of a tile may not bereferred to as a tile.

The bricks in a picture may also be arranged in a slice. A slice may bean integer number of bricks of a picture that may be exclusivelycontained in a single network abstraction layer (NAL) unit. In someexamples, a slice includes either a number of complete tiles or only aconsecutive sequence of complete bricks of one tile.

This disclosure may use “N×N” and “N by N” interchangeably to refer tothe sample dimensions of a block (such as a CU or other video block) interms of vertical and horizontal dimensions, e.g., 16×16 samples or 16by 16 samples. In general, a 16×16 CU will have 16 samples in a verticaldirection (y=16) and 16 samples in a horizontal direction (x=16).Likewise, an N×N CU generally has N samples in a vertical direction andN samples in a horizontal direction, where N represents a nonnegativeinteger value. The samples in a CU may be arranged in rows and columns.Moreover, CUs need not necessarily have the same number of samples inthe horizontal direction as in the vertical direction. For example, CUsmay comprise N×M samples, where M is not necessarily equal to N.

Video encoder 200 encodes video data for CUs representing predictionand/or residual information, and other information. The predictioninformation indicates how the CU is to be predicted in order to form aprediction block for the CU. The residual information generallyrepresents sample-by-sample differences between samples of the CU priorto encoding and the prediction block.

To predict a CU, video encoder 200 may generally form a prediction blockfor the CU through inter-prediction or intra-prediction.Inter-prediction generally refers to predicting the CU from data of apreviously coded picture, whereas intra-prediction generally refers topredicting the CU from previously coded data of the same picture. Toperform inter-prediction, video encoder 200 may generate the predictionblock using one or more motion vectors. Video encoder 200 may generallyperform a motion search to identify a reference block that closelymatches the CU, e.g., in terms of differences between the CU and thereference block. Video encoder 200 may calculate a difference metricusing a sum of absolute difference (SAD), sum of squared differences(SSD), mean absolute difference (MAD), mean squared differences (MSD),or other such difference calculations to determine whether a referenceblock closely matches the current CU. In some examples, video encoder200 may predict the current CU using uni-directional prediction orbi-directional prediction.

Some examples of JEM and VVC also provide an affine motion compensationmode, which may be considered an inter-prediction mode. In affine motioncompensation mode, video encoder 200 may determine two or more motionvectors that represent non-translational motion, such as zoom in or out,rotation, perspective motion, or other irregular motion types.

To perform intra-prediction, video encoder 200 may select anintra-prediction mode to generate the prediction block. Some examples ofJEM and VVC provide sixty-seven intra-prediction modes, includingvarious directional modes, as well as planar mode and DC mode. Ingeneral, video encoder 200 selects an intra-prediction mode thatdescribes neighboring samples to a current block (e.g., a block of a CU)from which to predict samples of the current block. Such samples maygenerally be above, above and to the left, or to the left of the currentblock in the same picture as the current block, assuming video encoder200 codes CTUs and CUs in raster scan order (left to right, top tobottom).

Video encoder 200 encodes data representing the prediction mode for acurrent block. For example, for inter-prediction modes, video encoder200 may encode data representing which of the various availableinter-prediction modes is used, as well as motion information for thecorresponding mode. For uni-directional or bi-directionalinter-prediction, for example, video encoder 200 may encode motionvectors using advanced motion vector prediction (AMVP) or merge mode.Video encoder 200 may use similar modes to encode motion vectors foraffine motion compensation mode.

Following prediction, such as intra-prediction or inter-prediction of ablock, video encoder 200 may calculate residual data for the block. Theresidual data, such as a residual block, represents sample by sampledifferences between the block and a prediction block for the block,formed using the corresponding prediction mode. Video encoder 200 mayapply one or more transforms to the residual block, to producetransformed data in a transform domain instead of the sample domain. Forexample, video encoder 200 may apply a discrete cosine transform (DCT),an integer transform, a wavelet transform, or a conceptually similartransform to residual video data. Additionally, video encoder 200 mayapply a secondary transform following the first transform, such as amode-dependent non-separable secondary transform (MDNSST), a signaldependent transform, a Karhunen-Loeve transform (KLT), or the like.Video encoder 200 produces transform coefficients following applicationof the one or more transforms.

As noted above, following any transforms to produce transformcoefficients, video encoder 200 may perform quantization of thetransform coefficients. Quantization generally refers to a process inwhich transform coefficients are quantized to possibly reduce the amountof data used to represent the transform coefficients, providing furthercompression. By performing the quantization process, video encoder 200may reduce the bit depth associated with some or all of the transformcoefficients. For example, video encoder 200 may round an n-bit valuedown to an m-bit value during quantization, where n is greater than m.In some examples, to perform quantization, video encoder 200 may performa bitwise right-shift of the value to be quantized.

Following quantization, video encoder 200 may scan the transformcoefficients, producing a one-dimensional vector from thetwo-dimensional matrix including the quantized transform coefficients.The scan may be designed to place higher energy (and therefore lowerfrequency) transform coefficients at the front of the vector and toplace lower energy (and therefore higher frequency) transformcoefficients at the back of the vector. In some examples, video encoder200 may utilize a predefined scan order to scan the quantized transformcoefficients to produce a serialized vector, and then entropy encode thequantized transform coefficients of the vector. In other examples, videoencoder 200 may perform an adaptive scan. After scanning the quantizedtransform coefficients to form the one-dimensional vector, video encoder200 may entropy encode the one-dimensional vector, e.g., according tocontext-adaptive binary arithmetic coding (CABAC). Video encoder 200 mayalso entropy encode values for syntax elements describing metadataassociated with the encoded video data for use by video decoder 300 indecoding the video data.

To perform CABAC, video encoder 200 may assign a context within acontext model to a symbol to be transmitted. The context may relate to,for example, whether neighboring values of the symbol are zero-valued ornot. The probability determination may be based on a context assigned tothe symbol.

Video encoder 200 may further generate syntax data, such as block-basedsyntax data, picture-based syntax data, and sequence-based syntax data,to video decoder 300, e.g., in a picture header, a block header, a sliceheader, or other syntax data, such as a sequence parameter set (SPS),picture parameter set (PPS), or video parameter set (VPS). Video decoder300 may likewise decode such syntax data to determine how to decodecorresponding video data.

In this manner, video encoder 200 may generate a bitstream includingencoded video data, e.g., syntax elements describing partitioning of apicture into blocks (e.g., CUs) and prediction and/or residualinformation for the blocks. Ultimately, video decoder 300 may receivethe bitstream and decode the encoded video data.

In general, video decoder 300 performs a reciprocal process to thatperformed by video encoder 200 to decode the encoded video data of thebitstream. For example, video decoder 300 may decode values for syntaxelements of the bitstream using CABAC in a manner substantially similarto, albeit reciprocal to, the CABAC encoding process of video encoder200. The syntax elements may define partitioning information of apicture into CTUs, and partitioning of each CTU according to acorresponding partition structure, such as a QTBT structure, to defineCUs of the CTU. The syntax elements may further define prediction andresidual information for blocks (e.g., CUs) of video data.

The residual information may be represented by, for example, quantizedtransform coefficients. Video decoder 300 may inverse quantize andinverse transform the quantized transform coefficients of a block toreproduce a residual block for the block. Video decoder 300 uses asignaled prediction mode (intra- or inter-prediction) and relatedprediction information (e.g., motion information for inter-prediction)to form a prediction block for the block. Video decoder 300 may thencombine the prediction block and the residual block (on asample-by-sample basis) to reproduce the original block. Video decoder300 may perform additional processing, such as performing a deblockingprocess to reduce visual artifacts along boundaries of the block.

In accordance with the techniques of this disclosure, video encoder 200and video decoder 300 may be configured to perform techniques inaccordance with joint coding of chroma residuals (JCCR). As describedabove and described in more detail below, in JCCR, the residualinformation for two chroma components may be combined to reduce theamount of information that is signaled. However, some techniques of JCCRmay not be used for lossless coding. In some examples of thisdisclosure, video encoder 200 and video decoder 300 may utilize JCCRtechniques but in such a way that the operations are lossless or atleast less lossy than other techniques of JCCR.

In some examples of this disclosure, video decoder 300 may receiveinformation for first residual samples for a first chroma component,wherein the first residual samples are based on a difference between afirst chroma block of the first chroma component and a first predictionblock of the first chroma component, determine intermediate samplesbased on the first residual samples, receive information for secondresidual samples for a second chroma component, wherein the secondresidual samples are based on a difference between a second chroma blockof the second chroma component and the intermediate samples, wherein thesecond chroma block and the first chroma block are associated together(e.g., part of the same CU), reconstruct the first chroma block based onthe first residual samples and the first prediction block, andreconstruct the second chroma block based on the second residual samplesand the intermediate samples.

In some examples, video decoder 300 may receive information for a set ofvalues, wherein the set values are generated based on first set ofresidual samples for a first chroma block of a first chroma componentand second set of residual samples for a second chroma block of a secondchroma component, wherein the first chroma block and the second chromablock are associated, determine first intermediate samples based on theset of values, determine second intermediate samples based on the set ofvalues, receive information for first residual samples, wherein thefirst residual samples are based on a difference between a first chromablock of the first chroma component and the first intermediate samples,receive information for second residual samples, wherein the secondresidual samples are based on a difference between a second chroma blockof the second chroma component and the second intermediate samples,reconstruct the first chroma block based on the first residual samplesand the first intermediate samples, and reconstruct the second chromablock based on the second residual samples and the second intermediatesamples.

In some examples, video encoder 200 may signal information for firstresidual samples for a first chroma component, wherein the firstresidual samples are based on a difference between a first chroma blockof the first chroma component and a first prediction block of the firstchroma component, determine intermediate samples based on the firstresidual samples, and signal information for second residual samples fora second chroma component, wherein the second residual samples are basedon a difference between a second chroma block of the second chromacomponent and the intermediate samples, and wherein the second chromablock and the first chroma block are associated together (e.g., part ofthe same CU).

In some examples, video encoder 200 may generate a set of values basedon a first set of residual samples for a first chroma component and asecond set of residual samples for a second chroma component, whereinthe first set of residual samples are based on a difference between afirst chroma block of the first chroma component and a first predictionblock and the second set of residual samples are based on a differencebetween a second chroma block for the second chroma component and asecond prediction block, wherein the first chroma block and the secondchroma block are associated, signal information for the set of values,determine first intermediate samples based on the set of values,determine second intermediate samples based on the set of values, signalinformation for first residual samples, wherein the first residualsamples are based on a difference between the first chroma block of thefirst chroma component and the first intermediate samples, and signalinformation for second residual samples, wherein the second residualsamples are based on a difference between the second chroma block of thesecond chroma component and the second intermediate samples.

This disclosure may generally refer to “signaling” certain information,such as syntax elements. The term “signaling” may generally refer to thecommunication of values for syntax elements and/or other data used todecode encoded video data. That is, video encoder 200 may signal valuesfor syntax elements in the bitstream. In general, signaling refers togenerating a value in the bitstream. As noted above, source device 102may transport the bitstream to destination device 116 substantially inreal time, or not in real time, such as might occur when storing syntaxelements to storage device 112 for later retrieval by destination device116.

FIGS. 2A and 2B are conceptual diagrams illustrating an example quadtreebinary tree (QTBT) structure 130, and a corresponding coding tree unit(CTU) 132. The solid lines represent quadtree splitting, and dottedlines indicate binary tree splitting. In each split (i.e., non-leaf)node of the binary tree, one flag is signaled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting in this example.For the quadtree splitting, there is no need to indicate the splittingtype, because quadtree nodes split a block horizontally and verticallyinto 4 sub-blocks with equal size. Accordingly, video encoder 200 mayencode, and video decoder 300 may decode, syntax elements (such assplitting information) for a region tree level of QTBT structure 130(i.e., the solid lines) and syntax elements (such as splittinginformation) for a prediction tree level of QTBT structure 130 (i.e.,the dashed lines). Video encoder 200 may encode, and video decoder 300may decode, video data, such as prediction and transform data, for CUsrepresented by terminal leaf nodes of QTBT structure 130.

In general, CTU 132 of FIG. 2B may be associated with parametersdefining sizes of blocks corresponding to nodes of QTBT structure 130 atthe first and second levels. These parameters may include a CTU size(representing a size of CTU 132 in samples), a minimum quadtree size(MinQTSize, representing a minimum allowed quadtree leaf node size), amaximum binary tree size (MaxBTSize, representing a maximum allowedbinary tree root node size), a maximum binary tree depth (MaxBTDepth,representing a maximum allowed binary tree depth), and a minimum binarytree size (MinBTSize, representing the minimum allowed binary tree leafnode size).

The root node of a QTBT structure corresponding to a CTU may have fourchild nodes at the first level of the QTBT structure, each of which maybe partitioned according to quadtree partitioning. That is, nodes of thefirst level are either leaf nodes (having no child nodes) or have fourchild nodes. The example of QTBT structure 130 represents such nodes asincluding the parent node and child nodes having solid lines forbranches. If nodes of the first level are not larger than the maximumallowed binary tree root node size (MaxBTSize), then the nodes can befurther partitioned by respective binary trees. The binary treesplitting of one node can be iterated until the nodes resulting from thesplit reach the minimum allowed binary tree leaf node size (MinBTSize)or the maximum allowed binary tree depth (MaxBTDepth). The example ofQTBT structure 130 represents such nodes as having dashed lines forbranches. The binary tree leaf node is referred to as a coding unit(CU), which is used for prediction (e.g., intra-picture or inter-pictureprediction) and transform, without any further partitioning. Asdiscussed above, CUs may also be referred to as “video blocks” or“blocks.”

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 (luma samples and two corresponding 64×64 chroma samples),the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, theMinBTSize (for both width and height) is set as 4, and the MaxBTDepth isset as 4. The quadtree partitioning is applied to the CTU first togenerate quad-tree leaf nodes. The quadtree leaf nodes may have a sizefrom 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If theleaf quadtree node is 128×128, the leaf quadtree node will not befurther split by the binary tree, because the size exceeds the MaxBTSize(i.e., 64×64, in this example). Otherwise, the leaf quadtree node willbe further partitioned by the binary tree. Therefore, the quadtree leafnode is also the root node for the binary tree and has the binary treedepth as 0. When the binary tree depth reaches MaxBTDepth (4, in thisexample), no further splitting is permitted. The binary tree node havinga width equal to MinBTSize (4, in this example) implies no furtherhorizontal splitting is permitted. Similarly, a binary tree node havinga height equal to MinBTSize implies no further vertical splitting ispermitted for that binary tree node. As noted above, leaf nodes of thebinary tree are referred to as CUs, and are further processed accordingto prediction and transform without further partitioning.

“Versatile Video Coding (Draft 6),” Joint Video Experts Team (WET) ofITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 15^(th) Meeting:Gothenburg, SE, 3-12 Jul. 2019, WET-02001-vD (hereinafter “VVC Draft 6”)supports a mode where the chroma residuals are coded jointly. The usage(activation) of a joint chroma coding mode is indicated by a TU-levelflag tu_joint_cbcr_residual_flag and the selected mode is implicitlyindicated by the chroma CBFs. The flag tu_joint_cbcr_residual_flag ispresent if either or both chroma CBFs for a TU are equal to 1. In thepicture parameter set (PPS) and slice header, chroma QP offset valuesare signalled for the joint chroma residual coding mode to differentiatefrom the usual chroma QP offset values signalled for regular chromaresidual coding mode. These chroma QP offset values are used to derivethe chroma QP values for those blocks coded using the joint chromaresidual coding mode. When a corresponding joint chroma coding mode(modes 2 in Table 1 below) is active in a TU, this chroma QP offset isadded to the applied luma-derived chroma QP during quantization anddecoding of that TU. For the other modes (modes 1 and 3 in Table 1below), the chroma QPs are derived in the same way as for conventionalCb or Cr blocks. The reconstruction process of the chroma residuals(resCb and resCr) from the transmitted transform blocks is depicted inTable 1. When this mode is activated, one single joint chroma residualblock (resJointC′[x][y] in Table 1) is signalled, and a residual blockfor Cb (resCb) and a residual block for Cr (resCr) are derivedconsidering information such as tu_cbf_cb, tu_cbf_cr, and CSign, whichis a sign value specified in the picture header as described in Helmrichet al. “CE7: Joint chroma residual coding with multiple modes (testsCE7-2.1, CE7-2.2),” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3and ISO/IEC JTC 1/SC 29/WG 11, 15^(th) Meeting: Gothenburg, SE, 3-12Jul. 2019, JVET-00105 and Helmrich et al. “Alternative configuration forjoint chroma residual coding” Joint Video Experts Team (JVET) of ITU-TSG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 15^(th) Meeting: Gothenburg,SE, 3-12 Jul. 2019, JVET-00543.

At the encoder side, the joint chroma components are derived asexplained in the following. Depending on the mode, resJointC isgenerated by video encoder 200 as shown in Table 1. The reconstructionsfor the chroma residuals for different modes performed by video decoder300 are shown in Table 2.

For all modes, the original residual at the encoder side is calculatedas: resCb[x][y]=origCb[x][y]−predCb[x][y], andresCr[x][y]=origCr[x][y]−predCr[x][y]. The residuals derived at thedecoder side res′Cb[x][y] and res′Cr[x][y], in general, are not the sameas resCb[x][y] and resCr[x][y], due to transform and quantization, anddue to joint representation.

Also, at the decoder side, the joint residual resJointC′ is notnecessarily the same, unless it is transform and quantization bypassed,as the generated joint residual resJointC by video encoder 200 due totransform/quantization. Even if resJointC is sent lossless (e.g.,transquant-bypass or transform skip with QP=4 for bitdepth 8), there maynot be a guarantee of lossless reconstruction of the chroma residualsdue to joint representation of residuals.

TABLE 1 Generation of joint chroma residual tu_cbf_cb tu_cbf_crGeneration of joint residual mode 1 0 resJointC[ x ][ y ] = 1 ( 4 *resCb[ x ][ y ] + 2 * CSign * resCr[ x ][ y ] ) / 5. 1 1 resJointC[ x ][y ] = 2 ( resCb[ x ][ y ] + CSign * resCr[ x ][ y ] ) / 2. 0 1resJointC[ x ][ y ] = 3 ( 4 * resCr[ x ][ y ] + 2 * CSign * resCb[ x ][y ] ) / 5.

TABLE 2 Reconstruction of chroma residuals. tu_cbf_cb tu_cbf_crreconstruction of Cb and Cr residuals mode 1 0 res′Cb[ x ][ y ] =resJointC′[ x ][ y ] 1 res′Cr[ x ][ y ] = ( CSign * resJointC′[ x ][ y ]) >> 1 1 1 res′Cb[ x ][ y ] = resJointC′[ x ][ y ] 2 res′Cr[ x ][ y ] =CSign * resJointC′[ x ][ y ] 0 1 res′Cb[ x ][ y ] = 3 ( CSign *resJointC′[ x ][ y ] ) >> 1 res′Cr[ x ][ y ] = resJointC′[ x ][ y ]

The reconstruction at the decoder side (e.g., by video decoder 300), forall modes, is as follows: recCb[x][y]=predCb[x][y]+res′Cb[x][y] andrecCr[x][y]=predCr[x][y]+res′Cr[x][y]. The three joint chroma codingmodes described above may be supported in intra predicted CUs. Fornon-intra CUs, only mode 2 may be supported. Hence, in non-intra CUs,the syntax element tu_joint_cbcr_residual_flag may only be present ifboth chroma cbfs are 1.

In Xu et al., “CE8-related: A SPS Level Flag for BDPCM and JCCR,” JointVideo Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG11, 15th Meeting: Gothenburg, SE, 3-12 Jul. 2019, JVET-00376, a sequenceparameter set (SPS) level flag has been added to controlenabling/disabling of joint-Cb-Cr for each video sequence. Accordingly,by setting this SPS flag to zero, an explicit way of disabling JCCR canbe carried out in VVC.

The following describes lossless coding functionality with VVC codec.VVC is also planned to support lossless coding, and currently losslesscoding is being investigated in an Ad-hoc group (AHG) and in coreexperiments. During JVET 16^(th) meeting cycle, AHG18 produced alossless VTM anchor which is based on the HEVC transform quantizationbypass and extended to new VVC tools with some normative andnon-normative changes. Subsequently, in Bross et al. “AHG 18: Enablinglossless coding with minimal impact on VVC design,” Joint Video ExpertsTeam (WET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 16^(th)Meeting: Geneva, CH, 1-11 Oct. 2019, JVET-P0606, an improved design forenabling lossless coding with minimal impact on VVC design was proposed.

However, there may be issues with using JCCR with lossless coding (or atleast less lossy coding than where JCCR was previously used). Forcurrent JVET activity of enabling lossless functionality, the JCCR toolis disabled from SPS level. This is due to the reason that, JCCR onlysignals one residual for both Cb and Cr components, and in general, withthis approach it may not be possible to have lossless coding of theresidual, and hence, lossless coding may not be available.

This disclosure describes several techniques for achieving losslesscoding using existing JCCR mode with minimal modification. The exampletechniques may be used together or separately. For ease, this disclosuredescribes examples with respect to technique one and technique two.However, the techniques should not be construed as being limited to twotechniques.

For the current JCCR method, the generation of the joint chroma residualat the encoder side is performed by a linear combination of Cb and Crresiduals. In general, the joint residual computed in this way is not alossless version of either Cb and Cr residuals.

In technique one, two residuals are signaled, resJointC and resJointC2,to achieve lossless coding (for original JCCR only resJointC issignaled). The encoder side (e.g., operations of video encoder 200) anddecoder side (e.g., operations of video decoder 300) processes are shownin Table 3. For convenience, the components Cb and Cr are defined asfirst and second components. For mode 1 and mode 2, Cb and Cr are thefirst and second components, respectively, and for mode 3, Cr and Cb arethe first and second components, respectively. As shown in table 3,resJointC is calculated by video encoder 200 in such a way that thelossless reconstruction of the first component is available, and in somecases, guaranteed. That is, video encoder 200 and video decoder 300 usea lossless version of either resCb or resCr (depending on the mode) asresJointC. The clipping bounds are (0, 2^(bitdePth)−1), where bitdepthis the internal (processing) codec operated bit depth.

In the second step, an intermediate step of reconstruction of the secondcomponent (rec1Cr/rec1Cb) is performed as per the decoding process ofthe original JCCR method with clipping. Subsequently, resJointC2 iscalculated by subtracting this intermediate reconstruction from theoriginal second component. Here, the clipping stage may ensure that theintermediate reconstruction values are inside the dynamic range, andsubsequently, clipping may also ensure that resJointC2 will have thesame dynamic range as other residuals (for example, resJointC etc.).

In order to be lossless coding, resJointC and resJointC2 should be codedin a lossless way. That can either be achieved using bypassing transformand quantization stage (same as HEVC lossless mode), or with TransformSkip with QP=4+2*(8−bitdepth), which makes the quantization stepsize=1,and due to no transform, lossless coding is achievable. In someexamples, QP offsets of the JCCR mode also need to be zero.

TABLE 3 Encoding and decoding process for technique one. tu_cbf_cbtu_cbf_cr Generation of joint residual mode 1 0 resJointC[x][y] =resCb[x][y] = origCb[x][y] − predCb[x][y] 1 rec1Cr[x][y] =Clip(predCr[x][y] + CSign*resJointC[x][y] >>1) resJointC2[x][y] =OrigCr[x][y] − rec1Cr[x][y] 1 1 resJointC[x][y] = resCb[x][y] =origCb[x][y] − predCb[x][y] 2 rec1Cr[x][y] = Clip(predCr[x][y] +CSign*resJointC[x][y]) resJointC2[x][y] = OrigCr[x][y] − rec1Cr[x][y] 01 resJointC[x][y] = resCr[x][y] = origCr[x][y] − predCr[x][y] 3rec1Cb[x][y] = Clip(predCb[x][y] + CSign*resJointC[x][y] >>1)resJointC2[x][y] = OrigCb[x][y] − rec1Cb[x][y] tu_cbf_cb tu_cbf_crreconstruction of Cb and Cr components mode 1 0 resCb[x][y] =resJointC[x][y], recCb[x][y] = predCb[x][y] + 1 resCb[x][y] rec1Cr[x][y]= Clip(predCr[x][y] + CSign*resJointC[x][y] >>1) recCr[x][y] =rec1Cr[x][y] + resJointC2[x][y] 1 1 resCb[x][y] = resJointC[x][y],recCb[x][y] = predCb[x][y] + 2 resCb[x][y] rec1Cr[x][y] =Clip(predCr[x][y] + CSign*resJointC[x][y]) recCr[x][y] = rec1Cr[x][y] +resJointC2[x][y] 0 1 resCr[x][y] = resJointC[x][y], recCr[x][y] =predCr[x][y] + 3 resCr[x][y] rec1Cb[x][y] = Clip(predCb[x][y] +CSign*resJointC[x][y] >>1) recCb[x][y] = rec1Cb[x][y] + resJointC2[x][y]

The following is an example with 2×2 residuals with 10 bit internalbitdepth. In this example, CSign=+1 and mode 1 is used.

The original Cb chroma block (origCb) is

$\quad\begin{bmatrix}{1020} & {1018} \\{1016} & {1014}\end{bmatrix}$

and the Cb prediction block (predCb) is

$\begin{bmatrix}{900} & {898} \\{896} & {894}\end{bmatrix}.$

Therefore, viaeo encoder 200 may determine the Cb residual block (resCb)as

$\begin{bmatrix}{120} & {120} \\{120} & {120}\end{bmatrix}.$

The original Ur chroma block (origCr) is

$\quad\begin{bmatrix}{1022} & {1020} \\{1018} & {1016}\end{bmatrix}$

and the Cr prediction block (predCb) is

$\begin{bmatrix}{972} & {970} \\{968} & {966}\end{bmatrix}.$

Therefore, video encoder 200 may determine the Cr residual block (resCr)as

$\begin{bmatrix}{50} & {50} \\{50} & {50}\end{bmatrix}.$

Video encoder 200 may determine resJointC, which is to be signaled, asresCb=origCb−predCb, which equals

$\begin{bmatrix}{120} & {120} \\{120} & {120}\end{bmatrix}.$

Video decoder 300 may reconstruct the Cb chroma block (recCb) asresJointC+predCb, which is

$= {{\begin{bmatrix}{900} & {898} \\{896} & {894}\end{bmatrix} + \begin{bmatrix}{120} & {120} \\{120} & {120}\end{bmatrix}} = {\begin{bmatrix}{1020} & {1018} \\{1016} & {1014}\end{bmatrix}{\left( {= {OrigCb}} \right).}}}$

Video encoder 200 and video decoder 300 may both determine rec1Cr as

${{{Clip}\left( {{{Csign}^{*}\left( {{resJointC}\operatorname{>>}1} \right)} + {predCr}} \right)} = {{{Clip}\left( {{1^{*}\ \begin{bmatrix}{60} & {60} \\{60} & {60}\end{bmatrix}} + \ \begin{bmatrix}{972} & {970} \\{968} & {966}\end{bmatrix}} \right)} = {{Clip}\left( \begin{bmatrix}{1032} & {1030} \\{1028} & {1026}\end{bmatrix} \right)}}},$

which results in

$\begin{bmatrix}{1023} & {1023} \\{1023} & {1023}\end{bmatrix}.$

Video encoder 200 may determine resJointC2, which is to be signaled, asresJoint

${C\; 2} = {{{origCr} - {{rec}\; 1{Cr}}} = {\begin{bmatrix}{- 1} & {- 3} \\{- 5} & {- 7}\end{bmatrix}.}}$

Video decoder 300 may reconstruct the Cr chroma block (recCr) asresJointC2+rec1Cr, which equals origCr.

Although the method 1 builds on the original JCCR method with scalingfactor of ½ for the second component with respect to the first one, suchrestrictions may be removed for a more generalized approach with ascaling factor of k1 or k2 (in the range of (0,1) excluding the value of0 and 1), as shown in Table 3.1. The values k1 and k2 can be signaled inSPS or picture header or CTU level, or in TU level. The value of b is apositive integer, which may be predetermined, fixed or signaled in SPS,picture header, or CTU level.

TABLE 3.1 generalized version of technique one. tu_cbf_cb tu_cbf_crGeneration of joint residual mode 1 0 resJointC[x][y] = resCb[x][y] =origCb[x][y] − predCb[x][y] 1 rec1Cr[x][y] = Clip(predCr[x][y] +CSign*k1*resJointC[x][y] >> b) resJointC2[x][y] = OrigCr[x][y] −rec1Cr[x][y] 1 1 resJointC[x][y] = resCb[x][y] = origCb[x][y] −predCb[x][y] 2 rec1Cr[x][y] = Clip(predCr[x][y] +CSign*resJointC[x][y] >> b) resJointC2[x][y] = OrigCr[x][y] −rec1Cr[x][y] 0 1 resJointC[x][y] = resCr[x][y] = origCr[x][y] −predCr[x][y] 3 rec1Cb[x][y] = Clip(predCb[x][y] +CSign*k2*resJointC[x][y]) resJointC2[x][y] = OrigCb[x][y] − rec1Cb[x][y]tu_cbf_cb tu_cbf_cr reconstruction of Cb and Cr components mode 1 0resCb[x][y] = resJointC[x][y], recCb[x][y] = predCb[x][y] + resCb[x][y]1 rec1Cr[x][y] = Clip(predCr[x][y] + CSign*k1*resJointC[x][y] >> b)recCr[x][y] = rec1Cr[x][y] + resJointC2[x][y] 1 1 resCb[x][y] =resJointC[x][y], recCb[x][y] = predCb[x][y] + resCb[x][y] 2 rec1Cr[x][y]= Clip(predCr[x][y] + CSign*resJointC[x][y]) recCr[x][y] =rec1Cr[x][y] + resJointC2[x][y] 0 1 resCr[x][y] = resJointC[x][y],recCr[x][y] = predCr[x][y] + resCr[x][y] 3 rec1Cb[x][y] =Clip(predCb[x][y] + CSign*k2*resJointC[x][y] >> b) recCb[x][y] =rec1Cb[x][y] + resJointC2[x][y]

To implement the techniques of Tables 3 and 3.1 above, video encoder 200may, for example, be configured to determine first residual samples(e.g., recCb[x][y]=resJointC[x][y]) for a first chroma component of ablock of the video data, with the first residual samples correspondingto a difference between a first chroma block (e.g., recCb[x][y]) of thefirst chroma component and a first prediction block (e.g., predCb[x][y])of the first chroma component. Video encoder 200 determines intermediatereconstructed samples (e.g., rec1Cr[x][y]) based on the first residualsamples (e.g., resJointC[x][y]) and a second prediction block (e.g.,predCr[x][y]) of the second chroma component and determines secondresidual samples (e.g., resJointC2[x][y]) for the second chromacomponent of the block of video data, with the second residual samplescorresponding to a difference between a second chroma block (e.g.,recCr[x][y]) of the second chroma component and the intermediatereconstructed samples (e.g., rec1Cr[x][y]). Video encoder 200 mayoutput, in a bitstream of encoded video data, syntax elementsrepresenting the first residual samples and the second residual samples.

To implement the techniques of Tables 3 and 3.1 above, video decoder 300may, for example, be configured to receive information for firstresidual samples (e.g., resCb[x][y]=resJointC[x][y]) for a first chromacomponent of a block of the video data, with the first residual samplescorresponding to a difference between a first chroma block (e.g.,recCb[x][y]) of the first chroma component and a first prediction block(e.g., predCb[x][y]) of the first chroma component. Video decoder 300may determine intermediate reconstructed samples (e.g., rec1Cr[x][y])based on the first residual samples (e.g., resJointC[x][y]) and receiveinformation for second residual samples (e.g., resJointC2[x][y]) for asecond chroma component of the block of video data, with the secondresidual samples corresponding to a difference between a second chromablock (e.g., recCr[x][y]) of the second chroma component and theintermediate reconstructed samples (e.g., rec1Cr[x][y]). Video decoder300 may reconstruct the first chroma block based on the first residualsamples and the first prediction block (e.g.,recCb[x][y]=predCb[x][y]+resCb[x][y]) and reconstruct the second chromablock based on the second residual samples and the intermediatereconstructed samples (e.g., recCr[x][y]=rec1Cr[x][y]+resJointC2[x][y]).It should be understood that the examples above are described withrespect to modes 1 and 2, where the first chroma component is a Cbcomponent and the second chroma component is a Cr component. For mode 3,the first chroma component may be a C3 component and the second chromacomponent may be a Cb component.

In example technique one, two residuals are signaled resJointC andresJointC2. In example technique two, three residuals may be signaled.For example, in example technique two, three residuals are signaled,resJointC′, resJointC) and resJointC2, to achieve lossless coding (asdescribed above, for original JCCR only resJointC′ is signaled, and fortechnique one resJointC and resJointC2 are signaled).

The encoder side (e.g., performed by video encoder 200) and decoder side(e.g., performed by video decoder 300) processes are shown in Table 4and Table 5, respectively. For all the modes, the original residual atthe encoder side is calculated as:resCb[x][y]=origCb[x][y]−predCb[x][y], andresCr[x][y]=origCr[x][y]−predCr[x][y]. In some examples, resJointC(e.g., the uncompressed residual) is calculated in the same way as theoriginal JCCR process as shown previously in Table 1. However, usingonly joint residual resJointC′ (which for lossless coding equals toresJointC) for reconstructing the Cb and Cr component may not providelossless representation. Accordingly, the additional secondary residualsfor Cb and Cr, for lossless coding are derived, which are resJointC1 andresJointC2 respectively, as shown in Table 4. In some examples,resJointC′ does not necessarily have to be the same as resJointC, soeven a lossy coding of resJointC can be performed (transform and/orquantization).

Compared to technique one, technique two uses one extra residual (intotal three), but the benefit is that, resJointC′ can be a lossy versionof resJointC (e.g., transform and/or quantization can be performed),which can be better in terms of compression.

TABLE 4 Generation of joint chroma residual for technique two tu_cbf_cbtu_cbf_cr Generation of joint residual mode 1 0 resJointC[ x ][ y ] = (4 * resCb[ x ][ y ] + 2 * CSign * resCr[ x ][ y ] ) / 5. 1 rec1Cb[x][y]= Clip(predCb[x][y] + resJointC′[x][y] ) rec1Cr[x][y] =Clip(predCr[x][y] + CSign*resJointC′[x][y] >>1) 1 1 resJointC[ x ][ y ]= ( resCb[ x ][ y ] + CSign * resCr[ x ][ y ]) / 2. 2 rec1Cb[x][y] =Clip(predCb[x][y] + resJointC′[x][y] ) rec1Cr[x][y] =Clip(predCr[x][y] + CSign*resJointC′[x][y]) 0 1 resJointC[ x ][ y ] = (4 * resCr[ x ][ y ] + 2 * CSign * resCb[ x ][ y ] ) / 5. 3 rec1Cb[x][y]= Clip(predCb[x][y] + CSign*resJointC′[x][y]>>1 ) rec1Cr[x][y] =Clip(predCr[x][y] + resJointC′[x][y]) For all the modes resJoint1[x][y]= origCb[x][y] − rec1Cb[x][y] resJoint2[x][y] = origCr[x][y] −rec1Cr[x][y]

TABLE 5 Reconstruction of chroma components for technique two. tu_cbf_cbtu_cbf_cr reconstruction of Cb and Cr residuals mode 1 0 rec1Cb[x][y] =Clip(predCb[x][y] + 1 resJointC′[x][y] rec1Cr[x][y] =Clip(predCr[x][y] + CSign*resJointC′[x][y] >>1) 1 1 rec1Cb[x][y] =Clip(predCb[x][y] + 2 resJointC′[x][y] ) rec1Cr[x][y] =Clip(predCr[x][y] + CSign*resJointC′[x][y]) 0 1 rec1Cb[x][y] =Clip(predCb[x][y] + 3 CSign*resJointC′[x][y]>>1 ) rec1Cr[x][y] =Clip(predCr[x][y] + resJointC′[x][y]) For all the modes recCb[x][y] =resJoint1[x][y] + rec1Cb[x][y] recCr[x][y] = resJoint2[x][y] +rec1Cr[x][y]

Similar to technique one, technique two can be also extended for a moregeneralized approach with a scaling factor of k1 or k2 (in the range of(0,1) excluding the value of 0 and 1) as shown in Table 6 and Table 7.The values k1 and k2 can be signaled in SPS or picture header, CTUlevel, or in TU level.

TABLE 6 generalized version of technique 2 (encoder) tu_cbf_cb tu_cbf_crGeneration of joint residual mode 1 0 resJointC[ x ][ y ] = ( 4 * resCb[x ][ y ] + 4 * k1 * CSign * resCr[ x ][ y ] ) / 5. 1 rec1Cb[x][y] =Clip(predCb[x][y] + resJointC′[x][y] ) rec1Cr[x][y] =Clip(predCr[x][y] + CSign*k1*resJointC′[x][y]) 1 1 resJointC[ x ][ y ] =( resCb[ x ][ y ] + CSign * resCr[ x ][ y ] ) / 2. 2 rec1Cb[x][y] =Clip(predCb[x][y] + resJointC′[x][y] ) rec1Cr[x][y] =Clip(predCr[x][y] + CSign*resJointC′[x][y]) 0 1 resJointC[ x ][ y ] = (4 * resCr[ x ][ y ] + 4 * k2 * CSign * resCb[ x ][ y ] ) / 5. 3rec1Cb[x][y] = Clip(predCb[x][y] + CSign*k2*resJointC′[x][y])rec1Cr[x][y] = Clip(predCr[x][y] + resJointC′[x][y]) For all the modesresJoint1[x][y] = origCb[x][y] − rec1Cb[x][y] resJoint2[x][y] =origCr[x][y] − rec1Cr[x][y]

TABLE 7 generalized version of method 2 (decoder) tu_cbf_cb tu_cbf_crreconstruction of Cb and Cr residuals mode 1 0 rec1Cb[x][y] =Clip(predCb[x][y] + resJointC′[x][y] ) 1 rec1Cr[x][y] =Clip(predCr[x][y] + CSign*k1*resJointC′[x][y]) 1 1 rec1Cb[x][y] =Clip(predCb[x][y] + resJointC′[x][y] ) 2 rec1Cr[x][y] =Clip(predCr[x][y] + CSign*resJointC′[x][y]) 0 1 rec1Cb[x][y] =Clip(predCb[x][y] + CSign*k2*resJointC′[x][y]) 3 rec1Cr[x][y] =Clip(predCr[x][y] + resJointC′[x][y]) For all the modes recCb[x][y] =resJoint1[x][y] + rec1Cb[x][y] recCr[x][y] = resJoint2[x][y] +rec1Cr[x][y]

The following describes transform unit—(TU) level and HLS (high levelsyntax) level control. Either technique one or technique two can be usedfor lossy coding or lossless coding. Accordingly, a TU level flag mayindicate whether JCCR uses 1 residual (original JCCR) or more than 1residual (modified JCCR, which may be technique one or technique two).In some examples, additional signaling may be used to specify betweentechnique one and technique two. An SPS/picture header level control canalso be provided to disable the option of using modified JCCR, so thatthe TU level flag for modified JCCR can be avoided. In some examples,modified JCCR can be used to replace original JCCR.

FIG. 3 is a block diagram illustrating an example video encoder 200 thatmay perform the techniques of this disclosure. FIG. 3 is provided forpurposes of explanation and should not be considered limiting of thetechniques as broadly exemplified and described in this disclosure. Forpurposes of explanation, this disclosure describes video encoder 200 inthe context of video coding standards such as the HEVC video codingstandard and the H.266 (VVC) video coding standard in development.However, the techniques of this disclosure are not limited to thesevideo coding standards, and are applicable generally to video encodingand decoding.

In the example of FIG. 3, video encoder 200 includes video data memory230, mode selection unit 202, residual generation unit 204, transformprocessing unit 206, quantization unit 208, inverse quantization unit210, inverse transform processing unit 212, reconstruction unit 214,filter unit 216, decoded picture buffer (DPB) 218, and entropy encodingunit 220. Any or all of video data memory 230, mode selection unit 202,residual generation unit 204, transform processing unit 206,quantization unit 208, inverse quantization unit 210, inverse transformprocessing unit 212, reconstruction unit 214, filter unit 216, DPB 218,and entropy encoding unit 220 may be implemented in one or moreprocessors or in processing circuitry. For instance, the units of videoencoder 200 may be implemented as one or more circuits or logic elementsas part of hardware circuitry, or as part of a processor, ASIC, or FPGA.Moreover, video encoder 200 may include additional or alternativeprocessors or processing circuitry to perform these and other functions.

Video data memory 230 may store video data to be encoded by thecomponents of video encoder 200. Video encoder 200 may receive the videodata stored in video data memory 230 from, for example, video source 104(FIG. 1). DPB 218 may act as a reference picture memory that storesreference video data for use in prediction of subsequent video data byvideo encoder 200. Video data memory 230 and DPB 218 may be formed byany of a variety of memory devices, such as dynamic random access memory(DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM),resistive RAM (RRAM), or other types of memory devices. Video datamemory 230 and DPB 218 may be provided by the same memory device orseparate memory devices. In various examples, video data memory 230 maybe on-chip with other components of video encoder 200, as illustrated,or off-chip relative to those components.

In this disclosure, reference to video data memory 230 should not beinterpreted as being limited to memory internal to video encoder 200,unless specifically described as such, or memory external to videoencoder 200, unless specifically described as such. Rather, reference tovideo data memory 230 should be understood as reference memory thatstores video data that video encoder 200 receives for encoding (e.g.,video data for a current block that is to be encoded). Memory 106 ofFIG. 1 may also provide temporary storage of outputs from the variousunits of video encoder 200.

The various units of FIG. 3 are illustrated to assist with understandingthe operations performed by video encoder 200. The units may beimplemented as fixed-function circuits, programmable circuits, or acombination thereof. Fixed-function circuits refer to circuits thatprovide particular functionality, and are preset on the operations thatcan be performed. Programmable circuits refer to circuits that can beprogrammed to perform various tasks, and provide flexible functionalityin the operations that can be performed. For instance, programmablecircuits may execute software or firmware that cause the programmablecircuits to operate in the manner defined by instructions of thesoftware or firmware. Fixed-function circuits may execute softwareinstructions (e.g., to receive parameters or output parameters), but thetypes of operations that the fixed-function circuits perform aregenerally immutable. In some examples, one or more of the units may bedistinct circuit blocks (fixed-function or programmable), and in someexamples, one or more of the units may be integrated circuits.

Video encoder 200 may include arithmetic logic units (ALUs), elementaryfunction units (EFUs), digital circuits, analog circuits, and/orprogrammable cores, formed from programmable circuits. In examples wherethe operations of video encoder 200 are performed using softwareexecuted by the programmable circuits, memory 106 (FIG. 1) may store theinstructions (e.g., object code) of the software that video encoder 200receives and executes, or another memory within video encoder 200 (notshown) may store such instructions.

Video data memory 230 is configured to store received video data. Videoencoder 200 may retrieve a picture of the video data from video datamemory 230 and provide the video data to residual generation unit 204and mode selection unit 202. Video data in video data memory 230 may beraw video data that is to be encoded.

Mode selection unit 202 includes a motion estimation unit 222, motioncompensation unit 224, and an intra-prediction unit 226. Mode selectionunit 202 may include additional functional units to perform videoprediction in accordance with other prediction modes. As examples, modeselection unit 202 may include a palette unit, an intra-block copy unit(which may be part of motion estimation unit 222 and/or motioncompensation unit 224), an affine unit, a linear model (LM) unit, or thelike.

Mode selection unit 202 generally coordinates multiple encoding passesto test combinations of encoding parameters and resultingrate-distortion values for such combinations. The encoding parametersmay include partitioning of CTUs into CUs, prediction modes for the CUs,transform types for residual data of the CUs, quantization parametersfor residual data of the CUs, and so on. Mode selection unit 202 mayultimately select the combination of encoding parameters havingrate-distortion values that are better than the other testedcombinations.

Video encoder 200 may partition a picture retrieved from video datamemory 230 into a series of CTUs, and encapsulate one or more CTUswithin a slice. Mode selection unit 202 may partition a CTU of thepicture in accordance with a tree structure, such as the QTBT structureor the quad-tree structure of HEVC described above. As described above,video encoder 200 may form one or more CUs from partitioning a CTUaccording to the tree structure. Such a CU may also be referred togenerally as a “video block” or “block.”

In general, mode selection unit 202 also controls the components thereof(e.g., motion estimation unit 222, motion compensation unit 224, andintra-prediction unit 226) to generate a prediction block for a currentblock (e.g., a current CU, or in HEVC, the overlapping portion of a PUand a TU). For inter-prediction of a current block, motion estimationunit 222 may perform a motion search to identify one or more closelymatching reference blocks in one or more reference pictures (e.g., oneor more previously coded pictures stored in DPB 218). In particular,motion estimation unit 222 may calculate a value representative of howsimilar a potential reference block is to the current block, e.g.,according to sum of absolute difference (SAD), sum of squareddifferences (SSD), mean absolute difference (MAD), mean squareddifferences (MSD), or the like. Motion estimation unit 222 may generallyperform these calculations using sample-by-sample differences betweenthe current block and the reference block being considered. Motionestimation unit 222 may identify a reference block having a lowest valueresulting from these calculations, indicating a reference block thatmost closely matches the current block.

Motion estimation unit 222 may form one or more motion vectors (MVs)that defines the positions of the reference blocks in the referencepictures relative to the position of the current block in a currentpicture. Motion estimation unit 222 may then provide the motion vectorsto motion compensation unit 224. For example, for uni-directionalinter-prediction, motion estimation unit 222 may provide a single motionvector, whereas for bi-directional inter-prediction, motion estimationunit 222 may provide two motion vectors. Motion compensation unit 224may then generate a prediction block using the motion vectors. Forexample, motion compensation unit 224 may retrieve data of the referenceblock using the motion vector. As another example, if the motion vectorhas fractional sample precision, motion compensation unit 224 mayinterpolate values for the prediction block according to one or moreinterpolation filters. Moreover, for bi-directional inter-prediction,motion compensation unit 224 may retrieve data for two reference blocksidentified by respective motion vectors and combine the retrieved data,e.g., through sample-by-sample averaging or weighted averaging.

As another example, for intra-prediction, or intra-prediction coding,intra-prediction unit 226 may generate the prediction block from samplesneighboring the current block. For example, for directional modes,intra-prediction unit 226 may generally mathematically combine values ofneighboring samples and populate these calculated values in the defineddirection across the current block to produce the prediction block. Asanother example, for DC mode, intra-prediction unit 226 may calculate anaverage of the neighboring samples to the current block and generate theprediction block to include this resulting average for each sample ofthe prediction block.

Mode selection unit 202 provides the prediction block to residualgeneration unit 204. Residual generation unit 204 receives a raw,unencoded version of the current block from video data memory 230 andthe prediction block from mode selection unit 202. Residual generationunit 204 calculates sample-by-sample differences between the currentblock and the prediction block. The resulting sample-by-sampledifferences define a residual block for the current block. In someexamples, residual generation unit 204 may also determine differencesbetween sample values in the residual block to generate a residual blockusing residual differential pulse code modulation (RDPCM). In someexamples, residual generation unit 204 may be formed using one or moresubtractor circuits that perform binary subtraction.

In examples where mode selection unit 202 partitions CUs into PUs, eachPU may be associated with a luma prediction unit and correspondingchroma prediction units. Video encoder 200 and video decoder 300 maysupport PUs having various sizes. As indicated above, the size of a CUmay refer to the size of the luma coding block of the CU and the size ofa PU may refer to the size of a luma prediction unit of the PU. Assumingthat the size of a particular CU is 2N×2N, video encoder 200 may supportPU sizes of 2N×2N or N×N for intra prediction, and symmetric PU sizes of2N×2N, 2N×N, N×2N, N×N, or similar for inter prediction. Video encoder200 and video decoder 300 may also support asymmetric partitioning forPU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N for inter prediction.

In examples where mode selection unit 202 does not further partition aCU into PUs, each CU may be associated with a luma coding block andcorresponding chroma coding blocks. As above, the size of a CU may referto the size of the luma coding block of the CU. The video encoder 200and video decoder 300 may support CU sizes of 2N×2N, 2N×N, or N×2N.

For other video coding techniques such as an intra-block copy modecoding, an affine-mode coding, and linear model (LM) mode coding, as fewexamples, mode selection unit 202, via respective units associated withthe coding techniques, generates a prediction block for the currentblock being encoded. In some examples, such as palette mode coding, modeselection unit 202 may not generate a prediction block, and insteadgenerate syntax elements that indicate the manner in which toreconstruct the block based on a selected palette. In such modes, modeselection unit 202 may provide these syntax elements to entropy encodingunit 220 to be encoded.

As described above, residual generation unit 204 receives the video datafor the current block and the corresponding prediction block. Residualgeneration unit 204 then generates a residual block for the currentblock. To generate the residual block, residual generation unit 204calculates sample-by-sample differences between the prediction block andthe current block.

Transform processing unit 206 applies one or more transforms to theresidual block to generate a block of transform coefficients (referredto herein as a “transform coefficient block”). Transform processing unit206 may apply various transforms to a residual block to form thetransform coefficient block. For example, transform processing unit 206may apply a discrete cosine transform (DCT), a directional transform, aKarhunen-Loeve transform (KLT), or a conceptually similar transform to aresidual block. In some examples, transform processing unit 206 mayperform multiple transforms to a residual block, e.g., a primarytransform and a secondary transform, such as a rotational transform. Insome examples, transform processing unit 206 does not apply transformsto a residual block.

Quantization unit 208 may quantize the transform coefficients in atransform coefficient block, to produce a quantized transformcoefficient block. Quantization unit 208 may quantize transformcoefficients of a transform coefficient block according to aquantization parameter (QP) value associated with the current block.Video encoder 200 (e.g., via mode selection unit 202) may adjust thedegree of quantization applied to the transform coefficient blocksassociated with the current block by adjusting the QP value associatedwith the CU. Quantization may introduce loss of information, and thus,quantized transform coefficients may have lower precision than theoriginal transform coefficients produced by transform processing unit206.

Inverse quantization unit 210 and inverse transform processing unit 212may apply inverse quantization and inverse transforms to a quantizedtransform coefficient block, respectively, to reconstruct a residualblock from the transform coefficient block. Reconstruction unit 214 mayproduce a reconstructed block corresponding to the current block (albeitpotentially with some degree of distortion) based on the reconstructedresidual block and a prediction block generated by mode selection unit202. For example, reconstruction unit 214 may add samples of thereconstructed residual block to corresponding samples from theprediction block generated by mode selection unit 202 to produce thereconstructed block.

Filter unit 216 may perform one or more filter operations onreconstructed blocks. For example, filter unit 216 may performdeblocking operations to reduce blockiness artifacts along edges of CUs.Operations of filter unit 216 may be skipped, in some examples.

Video encoder 200 stores reconstructed blocks in DPB 218. For instance,in examples where operations of filter unit 216 are not needed,reconstruction unit 214 may store reconstructed blocks to DPB 218. Inexamples where operations of filter unit 216 are needed, filter unit 216may store the filtered reconstructed blocks to DPB 218. Motionestimation unit 222 and motion compensation unit 224 may retrieve areference picture from DPB 218, formed from the reconstructed (andpotentially filtered) blocks, to inter-predict blocks of subsequentlyencoded pictures. In addition, intra-prediction unit 226 may usereconstructed blocks in DPB 218 of a current picture to intra-predictother blocks in the current picture.

In general, entropy encoding unit 220 may entropy encode syntax elementsreceived from other functional components of video encoder 200. Forexample, entropy encoding unit 220 may entropy encode quantizedtransform coefficient blocks from quantization unit 208. As anotherexample, entropy encoding unit 220 may entropy encode prediction syntaxelements (e.g., motion information for inter-prediction or intra-modeinformation for intra-prediction) from mode selection unit 202. Entropyencoding unit 220 may perform one or more entropy encoding operations onthe syntax elements, which are another example of video data, togenerate entropy-encoded data. For example, entropy encoding unit 220may perform a context-adaptive variable length coding (CAVLC) operation,a CABAC operation, a variable-to-variable (V2V) length coding operation,a syntax-based context-adaptive binary arithmetic coding (SBAC)operation, a Probability Interval Partitioning Entropy (PIPE) codingoperation, an Exponential-Golomb encoding operation, or another type ofentropy encoding operation on the data. In some examples, entropyencoding unit 220 may operate in bypass mode where syntax elements arenot entropy encoded.

Video encoder 200 may output a bitstream that includes the entropyencoded syntax elements needed to reconstruct blocks of a slice orpicture. In particular, entropy encoding unit 220 may output thebitstream.

The operations described above are described with respect to a block.Such description should be understood as being operations for a lumacoding block and/or chroma coding blocks. As described above, in someexamples, the luma coding block and chroma coding blocks are luma andchroma components of a CU. In some examples, the luma coding block andthe chroma coding blocks are luma and chroma components of a PU.

In some examples, operations performed with respect to a luma codingblock need not be repeated for the chroma coding blocks. As one example,operations to identify a motion vector (MV) and reference picture for aluma coding block need not be repeated for identifying a MV andreference picture for the chroma blocks. Rather, the MV for the lumacoding block may be scaled to determine the MV for the chroma blocks,and the reference picture may be the same. As another example, theintra-prediction process may be the same for the luma coding block andthe chroma coding blocks.

Video encoder 200 represents an example of a device configured to encodevideo data including a memory configured to store video data, and one ormore processing units implemented in circuitry and configured to signalinformation for first residual samples for a first chroma component,wherein the first residual samples are based on a difference between afirst chroma block of the first chroma component and a first predictionblock of the first chroma component, determine intermediate samplesbased on the first residual samples, and signal information for secondresidual samples for a second chroma component, wherein the secondresidual samples are based on a difference between a second chroma blockof the second chroma component and the intermediate samples, wherein thesecond chroma block and the first chroma block are associated together.

The one or more processing units of video encoder 200 may also beconfigured to generate a set of values (e.g., resJointC) based on afirst set of residual samples (e.g., resCb) for a first chroma componentand a second set of residual samples (e.g., resCr) for a second chromacomponent, wherein the first set of residual samples are based on adifference between a first chroma block of the first chroma componentand a first prediction block and the second set of residual samples arebased on a difference between a second chroma block for the secondchroma component and a second prediction block, wherein the first chromablock and the second chroma block are associated (e.g., part of the sameCU), signal information for the set of values, signal information forthe set of values, determine first intermediate samples (e.g., rec1Cb)based on the set of values, determine second intermediate samples (e.g.,rec1Cr) based on the set of values, signal information for firstresidual samples (e.g., resJoint1), wherein the first residual samplesare based on a difference between the first chroma block of the firstchroma component and the first intermediate samples, and signalinformation for second residual samples (e.g., resJoint2), wherein thesecond residual samples are based on a difference between the secondchroma block of the second chroma component and the second intermediatesamples.

FIG. 4 is a block diagram illustrating an example video decoder 300 thatmay perform the techniques of this disclosure. FIG. 4 is provided forpurposes of explanation and is not limiting on the techniques as broadlyexemplified and described in this disclosure. For purposes ofexplanation, this disclosure describes video decoder 300 according tothe techniques of JEM, VVC, and HEVC. However, the techniques of thisdisclosure may be performed by video coding devices that are configuredto other video coding standards.

In the example of FIG. 4, video decoder 300 includes coded picturebuffer (CPB) memory 320, entropy decoding unit 302, predictionprocessing unit 304, inverse quantization unit 306, inverse transformprocessing unit 308, reconstruction unit 310, filter unit 312, anddecoded picture buffer (DPB) 314. Any or all of CPB memory 320, entropydecoding unit 302, prediction processing unit 304, inverse quantizationunit 306, inverse transform processing unit 308, reconstruction unit310, filter unit 312, and DPB 314 may be implemented in one or moreprocessors or in processing circuitry. For instance, the units of videodecoder 300 may be implemented as one or more circuits or logic elementsas part of hardware circuitry, or as part of a processor, ASIC, of FPGA.Moreover, video decoder 300 may include additional or alternativeprocessors or processing circuitry to perform these and other functions.

Prediction processing unit 304 includes motion compensation unit 316 andintra-prediction unit 318. Prediction processing unit 304 may includeadditional units to perform prediction in accordance with otherprediction modes. As examples, prediction processing unit 304 mayinclude a palette unit, an intra-block copy unit (which may form part ofmotion compensation unit 316), an affine unit, a linear model (LM) unit,or the like. In other examples, video decoder 300 may include more,fewer, or different functional components.

CPB memory 320 may store video data, such as an encoded video bitstream,to be decoded by the components of video decoder 300. The video datastored in CPB memory 320 may be obtained, for example, fromcomputer-readable medium 110 (FIG. 1). CPB memory 320 may include a CPBthat stores encoded video data (e.g., syntax elements) from an encodedvideo bitstream. Also, CPB memory 320 may store video data other thansyntax elements of a coded picture, such as temporary data representingoutputs from the various units of video decoder 300. DPB 314 generallystores decoded pictures, which video decoder 300 may output and/or useas reference video data when decoding subsequent data or pictures of theencoded video bitstream. CPB memory 320 and DPB 314 may be formed by anyof a variety of memory devices, such as DRAM, including SDRAM, MRAM,RRAM, or other types of memory devices. CPB memory 320 and DPB 314 maybe provided by the same memory device or separate memory devices. Invarious examples, CPB memory 320 may be on-chip with other components ofvideo decoder 300, or off-chip relative to those components.

Additionally or alternatively, in some examples, video decoder 300 mayretrieve coded video data from memory 120 (FIG. 1). That is, memory 120may store data as discussed above with CPB memory 320. Likewise, memory120 may store instructions to be executed by video decoder 300, whensome or all of the functionality of video decoder 300 is implemented insoftware to be executed by processing circuitry of video decoder 300.

The various units shown in FIG. 4 are illustrated to assist withunderstanding the operations performed by video decoder 300. The unitsmay be implemented as fixed-function circuits, programmable circuits, ora combination thereof. Similar to FIG. 3, fixed-function circuits referto circuits that provide particular functionality, and are preset on theoperations that can be performed. Programmable circuits refer tocircuits that can be programmed to perform various tasks, and provideflexible functionality in the operations that can be performed. Forinstance, programmable circuits may execute software or firmware thatcause the programmable circuits to operate in the manner defined byinstructions of the software or firmware. Fixed-function circuits mayexecute software instructions (e.g., to receive parameters or outputparameters), but the types of operations that the fixed-functioncircuits perform are generally immutable. In some examples, one or moreof the units may be distinct circuit blocks (fixed-function orprogrammable), and in some examples, one or more of the units may beintegrated circuits.

Video decoder 300 may include ALUs, EFUs, digital circuits, analogcircuits, and/or programmable cores formed from programmable circuits.In examples where the operations of video decoder 300 are performed bysoftware executing on the programmable circuits, on-chip or off-chipmemory may store instructions (e.g., object code) of the software thatvideo decoder 300 receives and executes.

Entropy decoding unit 302 may receive encoded video data from the CPBand entropy decode the video data to reproduce syntax elements.Prediction processing unit 304, inverse quantization unit 306, inversetransform processing unit 308, reconstruction unit 310, and filter unit312 may generate decoded video data based on the syntax elementsextracted from the bitstream.

In general, video decoder 300 reconstructs a picture on a block-by-blockbasis. Video decoder 300 may perform a reconstruction operation on eachblock individually (where the block currently being reconstructed, i.e.,decoded, may be referred to as a “current block”).

Entropy decoding unit 302 may entropy decode syntax elements definingquantized transform coefficients of a quantized transform coefficientblock, as well as transform information, such as a quantizationparameter (QP) and/or transform mode indication(s). Inverse quantizationunit 306 may use the QP associated with the quantized transformcoefficient block to determine a degree of quantization and, likewise, adegree of inverse quantization for inverse quantization unit 306 toapply. Inverse quantization unit 306 may, for example, perform a bitwiseleft-shift operation to inverse quantize the quantized transformcoefficients. Inverse quantization unit 306 may thereby form a transformcoefficient block including transform coefficients.

After inverse quantization unit 306 forms the transform coefficientblock, inverse transform processing unit 308 may apply one or moreinverse transforms to the transform coefficient block to generate aresidual block associated with the current block. For example, inversetransform processing unit 308 may apply an inverse DCT, an inverseinteger transform, an inverse Karhunen-Loeve transform (KLT), an inverserotational transform, an inverse directional transform, or anotherinverse transform to the transform coefficient block.

Furthermore, prediction processing unit 304 generates a prediction blockaccording to prediction information syntax elements that were entropydecoded by entropy decoding unit 302. For example, if the predictioninformation syntax elements indicate that the current block isinter-predicted, motion compensation unit 316 may generate theprediction block. In this case, the prediction information syntaxelements may indicate a reference picture in DPB 314 from which toretrieve a reference block, as well as a motion vector identifying alocation of the reference block in the reference picture relative to thelocation of the current block in the current picture. Motioncompensation unit 316 may generally perform the inter-prediction processin a manner that is substantially similar to that described with respectto motion compensation unit 224 (FIG. 3).

As another example, if the prediction information syntax elementsindicate that the current block is intra-predicted, intra-predictionunit 318 may generate the prediction block according to anintra-prediction mode indicated by the prediction information syntaxelements. Again, intra-prediction unit 318 may generally perform theintra-prediction process in a manner that is substantially similar tothat described with respect to intra-prediction unit 226 (FIG. 3).Intra-prediction unit 318 may retrieve data of neighboring samples tothe current block from DPB 314.

Reconstruction unit 310 may reconstruct the current block using theprediction block and the residual block. For example, reconstructionunit 310 may add samples of the residual block to corresponding samplesof the prediction block to reconstruct the current block.

Filter unit 312 may perform one or more filter operations onreconstructed blocks. For example, filter unit 312 may performdeblocking operations to reduce blockiness artifacts along edges of thereconstructed blocks. Operations of filter unit 312 are not necessarilyperformed in all examples.

Video decoder 300 may store the reconstructed blocks in DPB 314. Forinstance, in examples where operations of filter unit 312 are notperformed, reconstruction unit 310 may store reconstructed blocks to DPB314. In examples where operations of filter unit 312 are performed,filter unit 312 may store the filtered reconstructed blocks to DPB 314.As discussed above, DPB 314 may provide reference information, such assamples of a current picture for intra-prediction and previously decodedpictures for subsequent motion compensation, to prediction processingunit 304. Moreover, video decoder 300 may output decoded pictures (e.g.,decoded video) from DPB 314 for subsequent presentation on a displaydevice, such as display device 118 of FIG. 1.

In this manner, video decoder 300 represents an example of a videodecoding device including a memory configured to store video data, andone or more processing units implemented in circuitry and configured toreceive information for first residual samples for a first chromacomponent, wherein the first residual samples are based on a differencebetween a first chroma block of the first chroma component and a firstprediction block of the first chroma component, determine intermediatesamples based on the first residual samples, receive information forsecond residual samples for a second chroma component, wherein thesecond residual samples are based on a difference between a secondchroma block of the second chroma component and the intermediatesamples, and wherein the second chroma block and the first chroma blockare associated together, reconstruct the first chroma block based on thefirst residual samples and the first prediction block, and reconstructthe second chroma block based on the second residual samples and theintermediate samples.

The processing units of video decoder 300 may also be configured toreceive information for a set of values, wherein the set values aregenerated based on first set of residual samples (e.g., resCb) for afirst chroma block of a first chroma component and second set ofresidual samples (e.g., resCr) for a second chroma block of a secondchroma component, wherein the first chroma block and the second chromablock are associated (e.g., part of the same CU), determine firstintermediate samples based on the set of values (e.g., rec1Cb),determine second intermediate samples based on the set of values (e.g.,rec1Cr), receive information for first residual samples (e.g.,resJoint1), wherein the first residual samples are based on a differencebetween a first chroma block of the first chroma component and the firstintermediate samples, receive information for second residual samples(e.g., resJoint2), wherein the second residual samples are based on adifference between a second chroma block of the second chroma componentand the second intermediate samples, reconstruct the first chroma blockbased on the first residual samples and the first intermediate samples,and reconstruct the second chroma block based on the second residualsamples and the second intermediate samples.

FIG. 5 is a flowchart illustrating an example method for encoding acurrent block. The current block may comprise a current CU. Althoughdescribed with respect to video encoder 200 (FIGS. 1 and 3), it shouldbe understood that other devices may be configured to perform a methodsimilar to that of FIG. 5.

In this example, video encoder 200 initially predicts the current block(350). For example, video encoder 200 may form a prediction block forthe current block. Video encoder 200 may then calculate residualinformation for the current block (352). To calculate the residualinformation, video encoder 200 may calculate resJointC and resJointC2 asdescribed above (e.g., based on determining rec1Cr or rec1Cb) fortechnique one, or video encoder 200 may calculate resJointC, resJointC1,and resJointC2 as described above (e.g., based on determining rec1Cr andrec1Cb) for technique two.

Optionally (e.g., when lossless coding is not needed), Video encoder 200may then transform and quantize the residual information (354). Videoencoder 200 may scan the residual information (if arranged in blockform) (356). During the scan, or following the scan, video encoder 200may entropy encode the residual information (358). For example, videoencoder 200 may encode the residual information using CAVLC or CABAC.Video encoder 200 may then output (e.g., signal) the entropy encodedresidual information (360).

FIG. 6 is a flowchart illustrating an example method for decoding acurrent block of video data. The current block may comprise a currentCU. Although described with respect to video decoder 300 (FIGS. 1 and4), it should be understood that other devices may be configured toperform a method similar to that of FIG. 6.

Video decoder 300 may receive entropy encoded residual information suchas resJointC and resJointC2 for technique one or resJointC, resJointC1,and resJointC2 for technique two (370). Video decoder 300 may determineintermediate values (e.g., rec1Cb and rec1Cr for techniques one and twoas described above) (372). Video decoder 300 may predict the currentblock (374), e.g., using an intra- or inter-prediction mode as indicatedby the prediction information for the current block, to calculate aprediction block for the current block. Video decoder 300 may theninverse scan the residual information (if the residual information is inblock form) (376), to create a block of residual information. Wherelossless coding was not needed, and therefore shown in dashed lines,video decoder 300 may then inverse quantize and inverse transform thetransform coefficients to produce the residual information (378). Videodecoder 300 may ultimately decode the current block by combining theprediction block and the residual information (380).

FIG. 7 is a flowchart illustrating an example method for encoding acurrent block. The current block may be a current CU that includes afirst chroma component, a second chroma component, and potentially othercomponents such as a luma component. The current block may, for example,be coded in oner or both of a lossless coding mode or a joint coding ofchroma residuals mode. Although described with respect to video encoder200 (FIGS. 1 and 3), it should be understood that other devices may beconfigured to perform a method similar to that of FIG. 7.

For a first chroma component of a block of the video data, video decoder300 determines first residual samples that correspond to a differencebetween a first chroma block of the first chroma component and a firstprediction block of the first chroma component (400). Video decoder 300determines intermediate reconstructed samples based on the firstresidual samples and a second prediction block of the second chromacomponent (402). Video decoder 300 determines second residual samplesfor the second chroma component of the block of video data, wherein thesecond residual samples correspond to a difference between a secondchroma block of the second chroma component and the intermediatereconstructed samples (404). Video decoder 300 outputs, in a bitstreamof encoded video data, syntax elements representing the first residualsamples and the second residual samples (406). Video decoder 300 may,for example, output the bitstream for storage or transmission.

FIG. 8 is a flowchart illustrating an example method for decoding acurrent block of video data. The current block may be a current CU thatincludes a first chroma component, a second chroma component, andpotentially other components such as a luma component. The current blockmay, for example, be coded in oner or both of a lossless coding mode ora joint coding of chroma residuals mode. Although described with respectto video decoder 300 (FIGS. 1 and 4), it should be understood that otherdevices may be configured to perform a method similar to that of FIG. 8.

For a first chroma component of a block of the video data, video decoder300 receives information for first residual samples that correspond to adifference between a first chroma block of the first chroma componentand a first prediction block of the first chroma component (420). Videodecoder 300 may obtain the first prediction block utilizing intraprediction, inter prediction, or some other such prediction mode.

Video decoder 300 determines intermediate reconstructed samples based onthe first residual samples (422). To determine the intermediatereconstructed samples, video decoder 300 may, for example, determine theintermediate reconstructed samples based on the first residual samplesand a second prediction block of the second chroma component. Todetermine the intermediate reconstructed samples, video decoder 300 may,for example determine the intermediate reconstructed samples, e.g.,rec1Cr[x][y] as described above, according to the modes of Tables 3,3.1, or 4-6 above.

For a second chroma component of the block of video data, video decoder300 receives information for second residual samples that correspond toa difference between a second chroma block of the second chromacomponent and the intermediate reconstructed samples (424). As theintermediate reconstructed samples may typically be relatively close tothe actual values for the second chroma block, the second residualsamples can typically be coded with relatively few bits.

Video decoder 300 reconstructs the first chroma block based on the firstresidual samples and the first prediction block (426). To reconstructthe first chroma block based on the first residual samples and the firstprediction block, video decoder 300 may for example add the firstresidual samples to the first prediction block. Video decoder 300reconstructs the second chroma block based on the second residualsamples and the intermediate reconstructed samples (428). To reconstructthe second chroma block based on the second residual samples and theintermediate reconstructed samples, video decoder 300 may, for example,add values of the second residual samples to corresponding values of theintermediate reconstructed samples. In instances where video decoder 300is implementing lossless coding, the reconstructed first chroma blockand reconstructed second chroma block may exactly match thecorresponding original first chroma block and second chroma blockwithout a need for any sort of filtering operations.

Video decoder 300 outputs decoded video data that includes thereconstructed first chroma block and the reconstructed second chromablock (430). Video decoder 300 may, for example, output the decodedvideo for display, output the decoded video data for long term storageto a non-volatile storage medium, or output the decoded video data to abuffer or other short term memory for use in the decoding of subsequentpictures of video.

The following clauses describe aspects of video encoder 200 and/or videodecoder 300 and techniques that can be implemented by video encoder 200and/or video decoder 300.

Aspect 1: A method of decoding video data includes receiving informationfor first residual samples for a first chroma component, wherein thefirst residual samples are based on a difference between a first chromablock of the first chroma component and a first prediction block of thefirst chroma component; determining intermediate samples based on thefirst residual samples; receiving information for second residualsamples for a second chroma component, wherein the second residualsamples are based on a difference between a second chroma block of thesecond chroma component and the intermediate samples, wherein the secondchroma block and the first chroma block are associated together;reconstructing the first chroma block based on the first residualsamples and the first prediction block; and reconstructing the secondchroma block based on the second residual samples and the intermediatesamples.

Aspect 2: The method of aspect 1, wherein the first residual samplesrepresent the difference between the first chroma block of the firstchroma component and the first prediction block of the first chromacomponent.

Aspect 3: The method of any of aspects 1 and 2, wherein determining theintermediate samples comprises determining the intermediate samplesbased on the first residual samples and a second prediction block of thesecond chroma component.

Aspect 4: The method of any of aspects 1-3, wherein the first chromablock and the second chroma block are of a same coding unit (CU).

Aspect 5: The method of any of aspects 1-4, wherein determining theintermediate samples comprises one of: for a first mode,rec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]>>1) orClip(predCr[x][y]+CSign*k1*resJointC[x][y]), where rec1Cr[x][y]represents the intermediate samples, Clip is the clipping operation,predCr[x][y] represents samples of the second prediction block of thesecond chroma component, Csign is a sign value, resJointC[x][y]represents samples of the first residual samples, >> is the right-shiftor divide by 2 operation, and k1 is a pre-stored or signaled parameter;for a second mode,rec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]), where rec1Cr[x][y] represents the intermediate samples, Clip is the clippingoperation, predCr[x][y] represents samples of the second predictionblock of the second chroma component, Csign is a sign value, andresJointC[x][y] represents samples of the first residual samples; or fora third mode, rec1Cb[x][y]=Clip(predCb[x][y]+CSign*resJointC[x][y]>>1)or Clip(predCb[x][y] +CSign*k2*resJointC[x][y]), where rec1Cr[x][y]represents the intermediate samples, Clip is the clipping operation,predCb[x][y] represents samples of the second prediction block of thesecond chroma component, CSign is a signal value, resJointC[x][y]represents samples of the first residual samples, >> is the right-shiftor divide by two operation, and k2 is a pre-stored or signaledparameter.

Aspect 6: A method of encoding video data includes signaling informationfor first residual samples for a first chroma component, wherein thefirst residual samples are based on a difference between a first chromablock of the first chroma component and a first prediction block of thefirst chroma component; determining intermediate samples based on thefirst residual samples; and signaling information for second residualsamples for a second chroma component, wherein the second residualsamples are based on a difference between a second chroma block of thesecond chroma component and the intermediate samples, and wherein thesecond chroma block and the first chroma block are associated together.

Aspect 7: The method of aspect 6, wherein the first residual samplesrepresent the difference between the first chroma block of the firstchroma component and the first prediction block of the first chromacomponent.

Aspect 8: The method of any of aspects 6 and 7, wherein determining theintermediate samples comprises determining the intermediate samplesbased on the first residual samples and a second prediction block of thesecond chroma component.

Aspect 9: The method of any of aspects 6-8, wherein the first chromablock and the second chroma block are of a same coding unit (CU).

Aspect 10: The method of any of aspects 6-9, wherein determining theintermediate samples comprises one of: for a first mode,rec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]>>1) orClip(predCr[x][y]+CSign*k1*resJointC[x][y]), where rec1Cr[x][y]represents the intermediate samples, Clip is the clipping operation,predCr[x][y] represents samples of the second prediction block of thesecond chroma component, Csign is a sign value, resJointC[x][y]represents samples of the first residual samples, >> is the right-shiftor divide by 2 operation, and k1 is a pre-stored or signaled parameter;for a second mode,rec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]), where rec1Cr[x][y] represents the intermediate samples, Clip is the clippingoperation, predCr[x][y] represents samples of the second predictionblock of the second chroma component, Csign is a sign value, andresJointC[x][y] represents samples of the first residual samples; or fora third mode, rec1Cb[x][y]=Clip(predCb[x][y]+CSign*resJointC[x][y]>>1)or Clip(predCb[x][y] +CSign*k2*resJointC[x][y]), where rec1Cr[x][y]represents the intermediate samples, Clip is the clipping operation,predCb[x][y] represents samples of the second prediction block of thesecond chroma component, CSign is a signal value, resJointC[x][y]represents samples of the first residual samples, >> is the right-shiftor divide by two operation, and k2 is a pre-stored or signaledparameter.

Aspect 11: A method of decoding video data includes receivinginformation for a set of values, wherein the set values are generatedbased on first set of residual samples for a first chroma block of afirst chroma component and second set of residual samples for a secondchroma block of a second chroma component, wherein the first chromablock and the second chroma block are associated; determining firstintermediate samples based on the set of values; determining secondintermediate samples based on the set of values; receiving informationfor first residual samples, wherein the first residual samples are basedon a difference between the first chroma block of the first chromacomponent and the first intermediate samples; receiving information forsecond residual samples, wherein the second residual samples are basedon a difference between the second chroma block of the second chromacomponent and the second intermediate samples; reconstructing the firstchroma block based on the first residual samples and the firstintermediate samples; and reconstructing the second chroma block basedon the second residual samples and the second intermediate samples.

Aspect 12: The method of aspect 11, wherein determining the firstintermediate samples comprises determining the first intermediatesamples based on the set of values and a first prediction block of thefirst color component, and wherein determining the second intermediatesamples comprises determining the second intermediate samples based onthe set of values and a second prediction block of the second colorcomponent.

Aspect 13: The method of any of aspects 11 and 12, wherein the firstchroma block and the second chroma block are of a same coding unit (CU).

Aspect 14: The method of any of aspects 11-13, wherein determining firstintermediate samples based on the set of values and determining secondintermediate samples based on the set of values comprises one of: for afirst mode, rec1Cb[x][y]=Clip(predCb[x][y]+resJointC[x][y]) andrec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]>>1) orrec1Cr[x][y]=Clip(predCr[x][y]+CSign*k1*resJointC[x][y]), whererec1Cb[x][y] represents the first intermediate samples, Clip is theclipping operation, predCb[x][y] represents the first prediction block,resJointC[x][y] represents the set of values, rec1Cr[x][y] representssecond intermediate samples, predCr[x][y] represents the secondprediction block, CSign is a sign value, >> is the right-shift or divideby 2 operation, and k1 is a pre-stored or signaled parameter; for asecond mode, rec1Cb[x][y]=Clip(predCb[x][y]+resJointC[x][y]) andrec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]), whererec1Cb[x][y] represents the first intermediate samples, Clip is theclipping operation, predCb[x][y] represents the first prediction block,resJointC[x][y] represents the set of values, rec1Cr[x][y] representssecond intermediate samples, predCr[x][y] represents the secondprediction block, and CSign is a sign value; or for a third mode,rec1Cb[x][y]=Clip(predCb[x][y]+CSign*resJointC[x][y]>>1) andrec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]) orrec1Cb[x][y]=Clip(predCb[x][y]+CSign*k2*resJointC[x][y]), whererec1Cb[x][y] represents the first intermediate samples, Clip is theclipping operation, predCb[x][y] represents the first prediction block,resJointC[x][y] represents the set of values, rec1Cr[x][y] representssecond intermediate samples, predCr[x][y] represents the secondprediction block, CSign is a sign value, >> is the right-shift or divideby 2 operation, and k2 is a pre-stored or signaled parameter.

Aspect 15: A method of encoding video data includes generating a set ofvalues based on first set of residual samples for a first chromacomponent and second set of residual samples for a second chromacomponent, wherein the first set of residual samples are based on adifference between a first chroma block of the first chroma componentand a first prediction block and the second set of residual samples arebased on a difference between a second chroma block for the secondchroma component and a second prediction block, wherein the first chromablock and the second chroma block are associated; signaling informationfor the set of values; determining first intermediate samples based onthe set of values; determining second intermediate samples based on theset of values; signaling information for first residual samples, whereinthe first residual samples are based on a difference between the firstchroma block of the first chroma component and the first intermediatesamples; and signaling information for second residual samples, whereinthe second residual samples are based on a difference between the secondchroma block of the second chroma component and the second intermediatesamples.

Aspect 16: The method of aspect 15, wherein determining the firstintermediate samples comprises determining the first intermediatesamples based on the set of values and a first prediction block of thefirst color component, and wherein determining the second intermediatesamples comprises determining the second intermediate samples based onthe set of values and a second prediction block of the second colorcomponent.

Aspect 17: The method of any of aspects 15 and 16, wherein the firstchroma block and the second chroma block are of a same coding unit (CU).

Aspect 18: The method of any of aspects 15-17, wherein generating theset of values comprises one of for a first mode,resJointC[x][y]=(4*resCb[x][y]+2*CSign*resCr[x][y])/5 orresJointC[x][y]=(4*resCb[x][y]+4*k1*CSign*resCr[x][y])/5, whereresJointC[x][y] represents the set of values, resCb[x][y] represents thefirst set of residual samples, CSign is a sign value, resCr[x][y]represents the second set of residual samples, and k1 is a pre-stored orsignaled value; for a second mode,resJointC[x][y]=(resCb[x][y]+CSign*resCr[x][y])/2 orresJointC[x][y]=(resCb[x][y]+CSign*resCr[x][y])/2, where resJointC[x][y]represents the set of values, resCb[x][y] represents the first set ofresidual samples, CSign is a sign value, resCr[x][y] represents thesecond set of residual samples, and k1 is a pre-stored or signaledvalue; or for a third mode,resJointC[x][y]=(4*resCr[x][y]+2*CSign*resCb[x][y])/5 orresJointC[x][y]=(4*resCr[x][y]+4*k2*CSign*resCb[x][y])/5, whereresJointC[x][y] represents the set of values, resCr[x][y] represents thefirst set of residual samples, CSign is a sign value, resCb[x][y]represents the second set of residual samples, and k2 is a pre-stored orsignaled value.

Aspect 19: The method of any of aspects 15-18, wherein determining firstintermediate samples based on the set of values and determining secondintermediate samples based on the set of values comprises one of: for afirst mode, rec1Cb[x][y]=Clip(predCb[x][y]+resJointC[x][y]) andrec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]>>1) orrec1Cr[x][y]=Clip(predCr[x][y]+CSign*k1*resJointC[x][y]), whererec1Cb[x][y] represents the first intermediate samples, Clip is theclipping operation, predCb[x][y] represents the first prediction block,resJointC[x][y] represents the set of values, rec1Cr[x][y] representssecond intermediate samples, predCr[x][y] represents the secondprediction block, CSign is a sign value, >> is the right-shift or divideby 2 operation, and k1 is a pre-stored or signaled parameter; for asecond mode, rec1Cb[x][y]=Clip(predCb[x][y]+resJointC[x][y]) andrec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]), whererec1Cb[x][y] represents the first intermediate samples, Clip is theclipping operation, predCb[x][y] represents the first prediction block,resJointC[x][y] represents the set of values, rec1Cr[x][y] representssecond intermediate samples, predCr[x][y] represents the secondprediction block, and CSign is a sign value; or for a third mode,rec1Cb[x][y]=Clip(predCb[x][y]+CSign*resJointC[x][y]>>1) andrec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]) orrec1Cb[x][y]=Clip(predCb[x][y]+CSign*k2*resJointC[x][y]), where rec1Cb[x][y] represents the first intermediate samples, Clip is the clippingoperation, predCb[x][y] represents the first prediction block,resJointC[x][y] represents the set of values, rec1Cr[x][y] representssecond intermediate samples, predCr[x][y] represents the secondprediction block, CSign is a sign value, >> is the right-shift or divideby 2 operation, and k2 is a pre-stored or signaled parameter.

Aspect 20: A device for decoding video data includes a memory configuredto store video data; and processing circuitry, coupled to the memory,and configured to perform the method of any of claims 1-5 and 11-14.

Aspect 21: A device for encoding video data includes a memory configuredto store video data; and processing circuitry, coupled to the memory,and configured to perform the method of any of claims 6-10 and 15-19.

Aspect 22: The device of any of aspects 20 or 21, further comprising adisplay configured to display decoded video data.

Aspect 23: The device of any of aspects 20-22, wherein the devicecomprises one or more of a camera, a computer, a mobile device, abroadcast receiver device, or a set-top box.

Aspect 24: A computer-readable storage medium storing instructionsthereon that when executed cause one or more processors to perform themethod of any of aspects 1-5 or 11-14.

Aspect 25: A device for coding video data, the device comprising meansfor performing the method of any of aspects 1-5 or 11-14.

It is to be recognized that depending on the example, certain acts orevents of any of the techniques described herein can be performed in adifferent sequence, may be added, merged, or left out altogether (e.g.,not all described acts or events are necessary for the practice of thetechniques). Moreover, in certain examples, acts or events may beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors, rather than sequentially.

In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implementedin software, the functions may be stored on or transmitted over as oneor more instructions or code on a computer-readable medium and executedby a hardware-based processing unit. Computer-readable media may includecomputer-readable storage media, which corresponds to a tangible mediumsuch as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place toanother, e.g., according to a communication protocol. In this manner,computer-readable media generally may correspond to (1) tangiblecomputer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storagemedia may be any available media that can be accessed by one or morecomputers or one or more processors to retrieve instructions, codeand/or data structures for implementation of the techniques described inthis disclosure. A computer program product may include acomputer-readable medium.

By way of example, and not limitation, such computer-readable storagemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage, or other magnetic storage devices, flashmemory, or any other medium that can be used to store desired programcode in the form of instructions or data structures and that can beaccessed by a computer. Also, any connection is properly termed acomputer-readable medium. For example, if instructions are transmittedfrom a website, server, or other remote source using a coaxial cable,fiber optic cable, twisted pair, digital subscriber line (DSL), orwireless technologies such as infrared, radio, and microwave, then thecoaxial cable, fiber optic cable, twisted pair, DSL, or wirelesstechnologies such as infrared, radio, and microwave are included in thedefinition of medium. It should be understood, however, thatcomputer-readable storage media and data storage media do not includeconnections, carrier waves, signals, or other transitory media, but areinstead directed to non-transitory, tangible storage media. Disk anddisc, as used herein, includes compact disc (CD), laser disc, opticaldisc, digital versatile disc (DVD), floppy disk and Blu-ray disc, wheredisks usually reproduce data magnetically, while discs reproduce dataoptically with lasers. Combinations of the above should also be includedwithin the scope of computer-readable media.

Instructions may be executed by one or more processors, such as one ormore digital signal processors (DSPs), general purpose microprocessors,application specific integrated circuits (ASICs), field programmablegate arrays (FPGAs), or other equivalent integrated or discrete logiccircuitry. Accordingly, the terms “processor” and “processingcircuitry,” as used herein may refer to any of the foregoing structuresor any other structure suitable for implementation of the techniquesdescribed herein. In addition, in some aspects, the functionalitydescribed herein may be provided within dedicated hardware and/orsoftware modules configured for encoding and decoding, or incorporatedin a combined codec. Also, the techniques could be fully implemented inone or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide varietyof devices or apparatuses, including a wireless handset, an integratedcircuit (IC) or a set of ICs (e.g., a chip set). Various components,modules, or units are described in this disclosure to emphasizefunctional aspects of devices configured to perform the disclosedtechniques, but do not necessarily require realization by differenthardware units. Rather, as described above, various units may becombined in a codec hardware unit or provided by a collection ofinteroperative hardware units, including one or more processors asdescribed above, in conjunction with suitable software and/or firmware.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of decoding video data, the methodcomprising: receiving information for first residual samples for a firstchroma component of a block of the video data, wherein the firstresidual samples correspond to a difference between a first chroma blockof the first chroma component and a first prediction block of the firstchroma component; determining intermediate reconstructed samples basedon the first residual samples; receiving information for second residualsamples for a second chroma component of the block of video data,wherein the second residual samples correspond to a difference between asecond chroma block of the second chroma component and the intermediatereconstructed samples; reconstructing the first chroma block based onthe first residual samples and the first prediction block;reconstructing the second chroma block based on the second residualsamples and the intermediate reconstructed samples; and outputtingdecoded video data comprising the reconstructed first chroma block andthe reconstructed second chroma block.
 2. The method of claim 1, whereinthe block of the video data is coded in a lossless coding mode.
 3. Themethod of claim 1, further comprising: determining a second predictionblock of the second chroma component using at least one of intraprediction or inter prediction; and determining the intermediatereconstructed samples based on the first residual samples and the secondprediction block of the second chroma component.
 4. The method of claim1, wherein the first chroma block and the second chroma block are of asame coding unit (CU).
 5. The method of claim 1, wherein reconstructingthe second chroma block based on the second residual samples and theintermediate reconstructed samples comprises adding values of the secondresidual samples to corresponding values of the intermediatereconstructed samples.
 6. The method of claim 1, further comprising:determining that the block of the video data is encoded in a jointcoding of chroma residuals mode.
 7. The method of claim 1, whereinreconstructing the first chroma block based on the first residualsamples and the first prediction block comprises adding the firstreconstructed chroma block to the first prediction block of the firstchroma component to generate the first chroma block; reconstructing thesecond chroma block based on the second residual samples and theintermediate reconstructed samples comprises adding values of the secondresidual samples to corresponding values of the intermediatereconstructed samples and to a second prediction block of the secondchroma component to generate the second chroma block.
 8. The method ofclaim 1, wherein determining the intermediate reconstructed samplescomprises determining the intermediate reconstructed samples accordingto one of the following equations:rec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]>>1) orClip(predCr[x][y]+C Sign*k1*resJointC[x][y]), wherein rec1Cr[x][y]represents the intermediate reconstructed samples, Clip is a clippingoperation, predCr[x][y] represents samples of a second prediction blockof the second chroma component, Csign is a sign value, resJointC[x][y]represents samples of the first residual samples, >> is a right-shift ordivide by 2 operation, and k1 is a pre-stored or signaled parameter. 9.The method of claim 1, wherein determining the intermediatereconstructed samples comprises determining the intermediatereconstructed samples according to the equation:rec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]), wherein rec1Cr[x][y] represents the intermediate reconstructed samples, Clip is aclipping operation, predCr[x][y] represents samples of a secondprediction block of the second chroma component, Csign is a sign value,and resJointC[x][y] represents samples of the first residual samples.10. The method of claim 1, wherein determining the intermediatereconstructed samples comprises determining the intermediatereconstructed samples according to one of the following equations:rec1Cb[x][y]=Clip(predCb[x][y]+CSign*resJointC[x][y]>>1), orClip(predCb[x][y]+CSign*k2*resJointC[x][y]), wherein rec1Cr[x][y]represents the intermediate reconstructed samples, Clip is a clippingoperation, predCb[x][y] represents samples of a second prediction blockof the second chroma component, CSign is a signal value, resJointC[x][y]represents samples of the first residual samples, >> is a right-shift ordivide by two operation, and k2 is a pre-stored or signaled parameter.11. A method of encoding video data, the method comprising: determiningfirst residual samples for a first chroma component of a block of thevideo data, wherein the first residual samples correspond to adifference between a first chroma block of the first chroma componentand a first prediction block of the first chroma component; determiningintermediate reconstructed samples based on the first residual samplesand a second prediction block of the second chroma component;determining second residual samples for the second chroma component ofthe block of video data, wherein the second residual samples correspondto a difference between a second chroma block of the second chromacomponent and the intermediate reconstructed samples; and outputting, ina bitstream of encoded video data, syntax elements representing thefirst residual samples and the second residual samples.
 12. The methodof claim 11, wherein the block of the video data is coded in a losslesscoding mode.
 13. The method of claim 11, wherein the first chroma blockand the second chroma block are of a same coding unit (CU).
 14. Themethod of claim 11, wherein determining the intermediate reconstructedsamples based on the first residual samples and the second predictionblock of the second chroma component comprises adding values of thefirst residual samples to values of samples of the second predictionblock.
 15. The method of claim 11, further comprising: determining thatthe block of the video data is encoded in a joint coding of chromaresiduals mode.
 16. A device for decoding video data, the devicecomprising: a memory configured to store the video data; one or moreprocessors implemented in circuitry and configured to: receiveinformation for first residual samples for a first chroma component of ablock of the video data, wherein the first residual samples correspondto a difference between a first chroma block of the first chromacomponent and a first prediction block of the first chroma component;determine intermediate reconstructed samples based on the first residualsamples; receive information for second residual samples for a secondchroma component of the block of video data, wherein the second residualsamples correspond to a difference between a second chroma block of thesecond chroma component and the intermediate reconstructed samples;reconstruct the first chroma block based on the first residual samplesand the first prediction block; reconstruct the second chroma blockbased on the second residual samples and the intermediate reconstructedsamples; and output decoded video data comprising the reconstructedfirst chroma block and the reconstructed second chroma block.
 17. Thedevice of claim 16, wherein the block of the video data is coded in alossless coding mode.
 18. The device of claim 16, wherein the one ormore processors are further configured to determine a second predictionblock of the second chroma component using at least one of intraprediction or inter prediction; and determine the intermediatereconstructed samples based on the first residual samples and the secondprediction block of the second chroma component.
 19. The device of claim16, wherein the first chroma block and the second chroma block are of asame coding unit (CU).
 20. The device of claim 16, wherein toreconstruct the second chroma block based on the second residual samplesand the intermediate reconstructed samples, the one or more processorsare further configured to add values of the second residual samples tocorresponding values of the intermediate reconstructed samples.
 21. Thedevice of claim 16, wherein the one or more processors are furtherconfigured to: determine that the block of the video data is encoded ina joint coding of chroma residuals mode.
 22. The device of claim 16,wherein to reconstruct the first chroma block based on the firstresidual samples and the first prediction block, the one or moreprocessors are further configured to adding the first reconstructedchroma block to the first prediction block of the first chroma componentto generate the first chroma block; to reconstruct the second chromablock based on the second residual samples and the intermediatereconstructed samples, the one or more processors are further configuredto add values of the second residual samples to corresponding values ofthe intermediate reconstructed samples and to a second prediction blockof the second chroma component to generate the second chroma block. 23.The device of claim 16, wherein to determine the intermediatereconstructed samples, the one or more processors are further configuredto determine the intermediate reconstructed samples according to one ofthe following equations:rec1 Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]>>1) orClip(predCr[x][y]+C Sign*k1*resJointC[x][y]), wherein rec1 Cr[x][y]represents the intermediate reconstructed samples, Clip is a clippingoperation, predCr[x][y] represents samples of a second prediction blockof the second chroma component, Csign is a sign value, resJointC[x][y]represents samples of the first residual samples, >> is a right-shift ordivide by 2 operation, and k1 is a pre-stored or signaled parameter. 24.The device of claim 16, wherein to determine the intermediatereconstructed samples, the one or more processors are further configuredto determine the intermediate reconstructed samples according to one ofthe following equations:rec1Cr[x][y]=Clip(predCr[x][y]+CSign*resJointC[x][y]), whereinrec1Cr[x][y] represents the intermediate reconstructed samples, Clip isa clipping operation, predCr[x][y] represents samples of a secondprediction block of the second chroma component, Csign is a sign value,and resJointC[x][y] represents samples of the first residual samples.25. The device of claim 16, wherein to determine the intermediatereconstructed samples, the one or more processors are further configuredto determine the intermediate reconstructed samples according to one ofthe following equations:rec1Cb[x][y]=Clip(predCb[x][y]+CSign*resJointC[x][y]>>1), orClip(predCb[x][y]+CSign*k2*resJointC[x][y]), wherein rec1Cr[x][y]represents the intermediate reconstructed samples, Clip is a clippingoperation, predCb[x][y] represents samples of a second prediction blockof the second chroma component, CSign is a signal value, resJointC[x][y]represents samples of the first residual samples, >> is a right-shift ordivide by two operation, and k2 is a pre-stored or signaled parameter.26. The device of claim 16, further comprising: a display configured todisplay decoded video data.
 27. The device of claim 16, wherein thedevice comprises one or more of a camera, a computer, a mobile device, abroadcast receiver device, or a set-top box.
 28. A device of encodingvideo data, the device comprising: a memory configured to store videodata; one or more processors implemented in circuitry and configured to:determine first residual samples for a first chroma component of a blockof the video data, wherein the first residual samples correspond to adifference between a first chroma block of the first chroma componentand a first prediction block of the first chroma component; determineintermediate reconstructed samples based on the first residual samplesand a second prediction block of the second chroma component; determinesecond residual samples for the second chroma component of the block ofvideo data, wherein the second residual samples correspond to adifference between a second chroma block of the second chroma componentand the intermediate reconstructed samples; and output, in a bitstreamof encoded video data, syntax elements representing the first residualsamples and the second residual samples.
 29. The device of claim 28,wherein the block of the video data is coded in a lossless coding mode.30. The device of claim 28, wherein to determine the intermediatereconstructed samples based on the first residual samples and the secondprediction block of the second chroma component, the one or moreprocessors are further configured to add values of the first residualsamples to values of samples of the second prediction block.